diff --git a/few/_version.py b/few/_version.py
index 123755b..c65dc2f 100644
--- a/few/_version.py
+++ b/few/_version.py
@@ -6,4 +6,4 @@
 
 """
 
-__version__ = '0.0.44'
+__version__ = '0.0.45'
diff --git a/few/few.py b/few/few.py
index 76bc9a5..cd220fd 100644
--- a/few/few.py
+++ b/few/few.py
@@ -60,7 +60,7 @@ def __init__(self, population_size=50, generations=100,
                  scoring_function=None, disable_update_check=False,
                  elitism=True, boolean = False,classification=False,clean=False,
                  track_diversity=False,mdr=False,otype='f',c=True,
-                 weight_parents=True,operators=None):
+                 weight_parents=True,operators=None, lex_size=False):
                 # sets up GP.
 
         # Save params to be recalled later by get_params()
@@ -95,6 +95,7 @@ def __init__(self, population_size=50, generations=100,
         self.op_weight = op_weight
         self.max_stall = max_stall
         self.weight_parents = weight_parents
+        self.lex_size = lex_size
         self.seed_with_ml = seed_with_ml
         self.erc = erc
         self.random_state = check_random_state(random_state)
@@ -726,6 +727,9 @@ def main():
                         dest='WEIGHT_PARENTS',default=True,
                         help='Feature importance weights parent selection.')
 
+    parser.add_argument('--lex_size', action='store_true',dest='LEX_SIZE',default=False,
+                        help='Size mediated parent selection for lexicase survival.')
+
     parser.add_argument('-sel', action='store', dest='SEL',
                         default='epsilon_lexicase',
                         choices = ['tournament','lexicase','epsilon_lexicase',
@@ -857,7 +861,7 @@ def main():
                   fit_choice = args.FIT_CHOICE,boolean=args.BOOLEAN,
                   classification=args.CLASSIFICATION,clean = args.CLEAN,
                   track_diversity=args.TRACK_DIVERSITY,mdr=args.MDR,
-                  otype=args.OTYPE,c=args.c,
+                  otype=args.OTYPE,c=args.c, lex_size = args.LEX_SIZE,
                   weight_parents = args.WEIGHT_PARENTS,operators=args.OPS)
 
     learner.fit(training_features, training_labels)
diff --git a/few/lib/epsilon_lexicase.h b/few/lib/epsilon_lexicase.h
index e9e7b97..2583571 100644
--- a/few/lib/epsilon_lexicase.h
+++ b/few/lib/epsilon_lexicase.h
@@ -81,7 +81,7 @@ static random_device rd;
 static mt19937 gen(rd());
 //extern "C"
 void epsilon_lexicase(const ExtMat & F, int n, int d,
-  int num_selections,  ExtVec& locs)
+  int num_selections,  ExtVec& locs, bool lex_size, ExtVec& sizes)
 {
   // training cases
   // ExtMat T (F, n, d);
@@ -96,15 +96,28 @@ void epsilon_lexicase(const ExtMat & F, int n, int d,
   for (int i = 0; i<epsilon.size(); ++i)
     epsilon(i) = mad(F.col(i));
 
-
-
-  // individual locations
-  vector<int> ind_locs(n);
-  iota(ind_locs.begin(),ind_locs.end(),0);
+  vector<int> ind_locs;
+  if(lex_size){
+    //randomly select a size from sizes
+    int max_index = sizes.size();
+    int random_index = rand() % max_index;
+
+    // individual locations
+    int j=0;
+    for(int i=0;i<max_index;i++){
+      if(sizes[i]<=sizes[random_index])
+        ind_locs.push_back(i);
+    }
+    
+  }
+  else{
+    // individual locations
+    ind_locs.resize(n);
+    iota(ind_locs.begin(),ind_locs.end(),0);
+  }
 
   // temporary winner pool
   vector<int> winner;
-
   for (int i = 0; i<num_selections; ++i){
     //cout << "selection " << i << "\n";
     // perform selection
@@ -121,7 +134,7 @@ void epsilon_lexicase(const ExtMat & F, int n, int d,
       winner.resize(0);
       // minimum error on case
       double minfit;
-      for (int j = 0; j< can_locs.size(); ++j){
+      for (int j = 0; j<can_locs.size(); ++j){
         if (j==0 || F(can_locs[j],cases.back())<minfit )
           minfit = F(can_locs[j],cases.back());
         }
diff --git a/few/lib/few_lib.pyx b/few/lib/few_lib.pyx
index 89816c3..2ab4dab 100644
--- a/few/lib/few_lib.pyx
+++ b/few/lib/few_lib.pyx
@@ -2,24 +2,16 @@
 from eigency.core cimport *
 cimport numpy as np
 from libcpp.vector cimport vector
+from libcpp cimport bool
 
 cdef extern from "epsilon_lexicase.h":
      cdef void _epsilon_lexicase "epsilon_lexicase"(Map[ArrayXXd] & F, int n,
                                                     int d, int num_selections,
-                                                    Map[ArrayXi] & locs)
+                                                    Map[ArrayXi] & locs, bool lex_size,
+                                                    Map[ArrayXi] &sizes)
 
 # This will be exposed to Python
-def ep_lex(np.ndarray F, int n, int d, int num_selections, np.ndarray locs):
+def ep_lex(np.ndarray F, int n, int d, int num_selections, np.ndarray locs, bool lex_size,
+           np.ndarray sizes):
     return _epsilon_lexicase(Map[ArrayXXd](F), n, d, num_selections,
-                               Map[ArrayXi](locs))
-# WIP
-# cdef extern from "evaluation.h":
-#     cdef void _evaluate "evaluate"(node n, Map[ArrayXXd] & features,
-#                                    vector[Map[ArrayXd]]] stack_float,
-#                                    vector[Map[ArrayXb]] stack_bool)
-
-# def evaluate(node n, np.ndarray features, vector[np.ndarray] stack_float,
-#              vector[np.ndarray] stack_bool):
-#     return _evaluate(node n, Map[ArrayXXd](features),
-#                      vector[Map[ArrayXd]]](stack_float),
-#                      vector[Map[ArrayXb]](stack_bool))
+                               Map[ArrayXi](locs), lex_size, Map[ArrayXi](sizes))
diff --git a/few/selection.py b/few/selection.py
index 1106545..351f55f 100644
--- a/few/selection.py
+++ b/few/selection.py
@@ -24,8 +24,13 @@ def survival(self,parents,offspring,elite=None,elite_index=None,X=None,F=None,F_
             survivors, survivor_index = self.lexicase(parents + offspring, num_selections = len(parents), survival = True)
         elif self.sel == 'epsilon_lexicase':
             # survivors, survivor_index = self.epsilon_lexicase(parents + offspring, num_selections = len(parents), survival = True)
-            survivor_index = self.epsilon_lexicase(np.vstack((F,F_offspring)), num_selections = F.shape[0], survival = True)
-            survivors = [(parents+ offspring)[s] for s in survivor_index]
+            if self.lex_size:
+                sizes = [len(i.stack) for i in (parents + offspring)]
+                survivor_index = self.epsilon_lexicase(np.vstack((F,F_offspring)), sizes, num_selections = F.shape[0], survival = True)
+                survivors = [(parents+ offspring)[s] for s in survivor_index]
+            else:
+                survivor_index = self.epsilon_lexicase(np.vstack((F,F_offspring)), [], num_selections = F.shape[0], survival = True)
+                survivors = [(parents+ offspring)[s] for s in survivor_index]
         elif self.sel == 'deterministic_crowding':
             survivors, survivor_index = self.deterministic_crowding(parents,offspring,X,X_offspring)
         elif self.sel == 'random':
@@ -107,14 +112,17 @@ def lexicase(self,individuals, num_selections=None, epsilon = False, survival =
 
         return winners, locs
 
-    def epsilon_lexicase(self, F, num_selections=None, survival = False):
+    def epsilon_lexicase(self, F, sizes, num_selections=None, survival = False):
         """conducts epsilon lexicase selection for de-aggregated fitness vectors"""
         # pdb.set_trace()
         if self.c: # use c library
             # define c types
             locs = np.empty(num_selections,dtype='int32',order='F')
             # self.lib.epsilon_lexicase(F,F.shape[0],F.shape[1],num_selections,locs)
-            ep_lex(F,F.shape[0],F.shape[1],num_selections,locs)
+            if self.lex_size:
+                ep_lex(F,F.shape[0],F.shape[1],num_selections,locs,self.lex_size,np.array(sizes))
+            else:
+                ep_lex(F,F.shape[0],F.shape[1],num_selections,locs,self.lex_size,np.array([]))
             return locs
         else: # use python version
             if num_selections is None:
diff --git a/few/tests/test_selection.py b/few/tests/test_selection.py
index cf1e143..f40a857 100644
--- a/few/tests/test_selection.py
+++ b/few/tests/test_selection.py
@@ -103,3 +103,24 @@ def test_lexicase_survival_shapes():
         i.fitness_vec = np.random.rand(10,1)
     offspring,locs = few.lexicase(pop.individuals,num_selections=1,survival=True)
     assert len(offspring) == 1;
+
+def test_lex_size():
+    """test_selection.py: lex_size flag on/off"""
+
+    few = FEW(seed_with_ml=False,population_size=257, lex_size=True)
+
+    Fitness_mat = np.random.rand(257,10)
+    size_mat = np.random.randint(1,100,size=257)
+
+    locs = few.epsilon_lexicase(Fitness_mat,size_mat,num_selections=100,
+                                          survival=True)
+    assert len(locs) == 100
+
+    few = FEW(seed_with_ml=False,population_size=257, lex_size=False)
+
+    Fitness_mat = np.random.rand(257,10)
+    size_mat = np.random.rand(257,1)
+
+    locs = few.epsilon_lexicase(Fitness_mat,size_mat,num_selections=100,
+                                          survival=True)
+    assert len(locs) == 100