Multivariate analyses were used to examine cranial morphology among populations of gray squirrels (Sciurus carolinensis) in Illinois [USA]. Phenetic relationships of 379 adult specimens from 12 localities in Illinois and 2 reference areas are discussed. S. carolinensis appears to vary clinally in size. Specimens from northern and north-central Illinois are similar to the northern reference specimens (S. c. pennsylvanicus), whereas specimens from the remainder of Illinois beyond areas of intergradation resemble the southern reference specimens (S. c. carolinensis). Electrophoresis of proteins from blood and body tissues did not distinguish between specimens from distant localities.