Although body size in reptiles has important consequences for their ecology, it is quite variable in most species, presumably because growth in these animals is influenced by many factors. In this study, we analyse the effects of family membership and sex on first-year growth of garter snakes (Thamnophis elegans) held under identical conditions in the laboratory. Litters, but not the two sexes, differed significantly in mean size at birth. Larger snakes ate more food and had a higher absolute (but not relative) growth rate in length than smaller snakes. Thus, there was significant variation in length increase among litters, but not between the sexes. All else being equal, variance in size at birth could contribute significantly to variance in adult body size.