diff --git a/treetime/vcf_utils.py b/treetime/vcf_utils.py
index 1408488b..4fa9d5ba 100644
--- a/treetime/vcf_utils.py
+++ b/treetime/vcf_utils.py
@@ -411,6 +411,21 @@ def write_vcf(tree_dict, file_name, mask=None):#, compress=False):
     positions = tree_dict['positions']
     ploidy = tree_dict.get('metadata', {}).get('ploidy', 2)
     chrom_name = tree_dict.get('metadata', {}).get('chrom', '1')
+    sample_names = list(sequences.keys())
+    inferred_const_sites = set(tree_dict.get('inferred_const_sites', []))
+
+    # For every variable site in sequences, flip the format around so
+    # we can have fast lookups later on.
+    alleles = {}
+    num_samples = len(sample_names)
+    for idx, name in enumerate(sample_names):
+        for posn, allele in sequences[name].items():
+            if posn not in alleles:
+                alleles[posn] = np.zeros(num_samples, dtype='U')
+            alleles[posn][idx] = allele
+    # fill in reference
+    for posn,bases in alleles.items():
+        bases[bases==''] = ref[posn]
 
     def handleDeletions(i, pi, pos, ref, delete, pattern):
         refb = ref[pi]
@@ -475,7 +490,7 @@ def handleDeletions(i, pi, pos, ref, delete, pattern):
 
 
     #prepare the header of the VCF & write out
-    header=["#CHROM","POS","ID","REF","ALT","QUAL","FILTER","INFO","FORMAT"]+list(sequences.keys())
+    header=["#CHROM","POS","ID","REF","ALT","QUAL","FILTER","INFO","FORMAT"]+sample_names
 
     opn = gzip.open if file_name.endswith(('.gz', '.GZ')) else open
     out_file = opn(file_name, 'w')
@@ -508,28 +523,8 @@ def handleDeletions(i, pi, pos, ref, delete, pattern):
             i+=1
             continue
 
-        #try/except is much more efficient than 'if' statements for constructing patterns,
-        #as on average a 'variable' location will not be variable for any given sequence
-        pattern = []
-        #pattern2 gets the pattern at next position to check for upcoming deletions
-        #it's more efficient to get both here rather than loop through sequences twice!
-        pattern2 = []
-        for k,v in sequences.items():
-            try:
-                pattern.append(sequences[k][pi])
-            except KeyError:
-                pattern.append(ref[pi])
-
-            try:
-                pattern2.append(sequences[k][pi+1])
-            except KeyError:
-                try:
-                    pattern2.append(ref[pi+1])
-                except IndexError:
-                    pass
-
-        pattern = np.array(pattern).astype('U')
-        pattern2 = np.array(pattern2).astype('U')
+        pattern = alleles[pi] if pi in alleles else np.array([]).astype('U')
+        pattern2 = alleles[pi+1] if pi+1 in alleles else np.array([]).astype('U')
 
         #If a deletion here, need to gather up all bases, and position before
         if any(pattern == '-'):
@@ -564,17 +559,13 @@ def handleDeletions(i, pi, pos, ref, delete, pattern):
         for u in uniques:
             pattern[np.where(pattern==u)[0]] = str(j)
             j+=1
-        #Now convert these calls to a VCF format matching the ploidy.
-        #In case ploidy>1, we treat it as unphased ('/' as the separator)
-        #Note that this includes patterns of "." (no-calls)
-        calls = [ "/".join([j]*ploidy) for j in pattern ]
 
         #What if there's no variation at a variable site??
         #This can happen when sites are modified by TreeTime - see below.
         printPos = True
         if len(uniques)==0:
             #If we expect it (it was made constant by TreeTime), it's fine.
-            if 'inferred_const_sites' in tree_dict and pi in tree_dict['inferred_const_sites']:
+            if pi in inferred_const_sites:
                 explainedErrors += 1
                 printPos = False #and don't output position to the VCF
             else:
@@ -584,6 +575,10 @@ def handleDeletions(i, pi, pos, ref, delete, pattern):
         #Write it out - Increment positions by 1 so it's in VCF numbering
         #If no longer variable, and explained, don't write it out
         if printPos:
+            #Now convert these calls to a VCF format matching the ploidy.
+            #In case ploidy>1, we treat it as unphased ('/' as the separator)
+            #Note that this includes patterns of "." (no-calls)
+            calls = [ "/".join([j]*ploidy) for j in pattern ]
             output = [chrom_name, str(pos), ".", refb, ",".join(uniques), ".", "PASS", ".", "GT"] + calls
             vcfWrite.append("\t".join(output))