diff --git a/src/Deprecated80/Behavior.extension.st b/src/Deprecated80/Behavior.extension.st
new file mode 100644
index 00000000000..bb5431c77fc
--- /dev/null
+++ b/src/Deprecated80/Behavior.extension.st
@@ -0,0 +1,7 @@
+Extension { #name : #Behavior }
+
+{ #category : #'*Deprecated80' }
+Behavior >> numberOfInstanceVariables [
+	self deprecated: 'Use #instSize or `slots size` instead.' transformWith: '`@receiver numberOfInstanceVariables' -> '`@receiver instSize'.
+	^ self instSize
+]
diff --git a/src/Kernel-Tests/BehaviorTest.class.st b/src/Kernel-Tests/BehaviorTest.class.st
index 440563c473d..2d29f2e0861 100644
--- a/src/Kernel-Tests/BehaviorTest.class.st
+++ b/src/Kernel-Tests/BehaviorTest.class.st
@@ -110,6 +110,13 @@ BehaviorTest >> testIncludesMethod [
 	self deny: (LookupKey includesMethod: Point>>#x).
 ]
 
+{ #category : #metrics }
+BehaviorTest >> testInstSize [
+	self assert: Object instSize equals: 0.
+	self assert: Point instSize equals: 2.
+	self assert: Metaclass instSize equals: 1
+]
+
 { #category : #tests }
 BehaviorTest >> testIsAbstract [
 
@@ -179,13 +186,6 @@ BehaviorTest >> testMethodsWritingSlot [
 	self assert: numberViaSlot = numberViaIVar.
 ]
 
-{ #category : #metrics }
-BehaviorTest >> testNumberOfInstanceVariables [
-	self assert: Object numberOfInstanceVariables equals: 0.
-	self assert: Point numberOfInstanceVariables equals: 2.
-	self assert: Metaclass numberOfInstanceVariables equals: 1
-]
-
 { #category : #'tests - properties' }
 BehaviorTest >> testPropertyValueAtPut [
 	| testValue |
diff --git a/src/Kernel/Behavior.class.st b/src/Kernel/Behavior.class.st
index e4cb4724d5f..2a0aaf7e8d5 100644
--- a/src/Kernel/Behavior.class.st
+++ b/src/Kernel/Behavior.class.st
@@ -1359,12 +1359,6 @@ Behavior >> nonObsoleteClass [
 	^ self environment at: obsName asSymbol
 ]
 
-{ #category : #accessing }
-Behavior >> numberOfInstanceVariables [
-	^ self instVarNames size
-
-]
-
 { #category : #initialization }
 Behavior >> obsolete [
 	"nothing to be done"
diff --git a/src/Morphic-Widgets-FastTable/FTExampleClassInfoTableDataSource.class.st b/src/Morphic-Widgets-FastTable/FTExampleClassInfoTableDataSource.class.st
index 757d706e5a3..c85775db30a 100644
--- a/src/Morphic-Widgets-FastTable/FTExampleClassInfoTableDataSource.class.st
+++ b/src/Morphic-Widgets-FastTable/FTExampleClassInfoTableDataSource.class.st
@@ -72,7 +72,7 @@ FTExampleClassInfoTableDataSource >> nameColumn: column row: rowIndex [
 { #category : #accessing }
 FTExampleClassInfoTableDataSource >> numberOfInstanceVariablesColumn: column row: rowIndex [
 	^ FTCellMorph new 
-		addMorph: (self elementAt: rowIndex) numberOfInstanceVariables asStringMorph;
+		addMorph: (self elementAt: rowIndex) instSize asStringMorph;
 		yourself
 ]
 
diff --git a/src/Morphic-Widgets-FastTable/FTExamples.class.st b/src/Morphic-Widgets-FastTable/FTExamples.class.st
index cb82a708d36..f5962618f2a 100644
--- a/src/Morphic-Widgets-FastTable/FTExamples.class.st
+++ b/src/Morphic-Widgets-FastTable/FTExamples.class.st
@@ -611,7 +611,7 @@ FTExamples class >> exampleSortableTable2 [
 				yourself);
 		addColumn:
 			((FTColumn id: 'Number of instance variables')
-				sortableOnProperty: #numberOfInstanceVariables;
+				sortableOnProperty: #instSize;
 				yourself);
 		dataSource: FTExampleClassInfoTableDataSource new;
 		selectFirst;
@@ -645,7 +645,7 @@ FTExamples class >> exampleSortableTable3 [
 				yourself);
 		addColumn:
 			((FTColumn id: 'Number of instance variables')
-				sortableUsing: #numberOfInstanceVariables ascending , #name ascending; "Sort by number of inst var, if the same, sort by name."
+				sortableUsing: #instSize ascending , #name ascending; "Sort by number of inst var, if the same, sort by name."
 				yourself);
 		dataSource: FTExampleClassInfoTableDataSource new;
 		selectFirst;