Data on over 950 natural matings of red-sided garter snakes, Thamnophis sirtalis parietalis, in Manitoba revealed size-assortative pairing: large males tended to mate with large females, and small males with small females. Unlike previously reported cases of size-assortative mating, the causal mechanism in these snakes involved a size-related shift in active mate selection by males. In the field, courtship as well as mating was size assortative (albeit, with considerable scatter around the trend line). Staged trials in outdoor arenas showed that males of all sizes preferred to court large rather than small females, but this preference was stronger in large males. Males adjusted their courtship intensity in response to the numbers and sizes of females and competing males, but did not change their preferences With respect to female body size. Thus, size-assortative mating was not a direct consequence of large males excluding their smaller rivals from large females. Males may be selective courters in this species because they have a limited supply of sperm and mating plugs, and hence can copulate effectively only a few times within the mating season. Given intense competition from large males (which primarily court large females), small males may benefit from focusing on small females. Alternatively, small males may be less capable of inducing sexual receptivity from large females. Mark-recapture data confirmed that males grow rapidly from one year to the next. Thus, the size-related shift in male mate choice was due to an ontogenetic change rather than the existence of multiple male morphs differing in both body size and courtship preference.