Bergmann's rule is currently defined as a within-species tendency for increasing body size with increasing latitude or decreasing environmental temperature. This well-known ecogeographic pattern has been considered a general trend for all animals, yet support for Bergmann's rule has only been demonstrated for mammals and birds. Here we evaluate Bergmann's rule in two groups of reptiles: chelonians (turtles) and squamates (lizards and snakes). We perform both nonphylogenetic and phylogenetic analyses and show that chelonians follow Bergmann's rule (19 of 23 species increase in size with latitude: 14 of 15 species decrease in size with temperature), whereas squamates follow the converse to Bergmann's rule (61 of 83 species decrease in size with latitude; 40 of 56 species increase in size with temperature). Size patterns of chelonians are significant using both nonphylogenetic and phylogenetic methods, whereas only the nonphylogenetic analyses are significant for squamates. These trends are consistent among major groups of chelonians and squamates for which data are available. This is the first study to document the converse to Bergmann's rule in any major animal group as well as the first to show Bergmann's rule in a major group of ectotherms. The traditional explanation for Bergmann's rule is that larger endothermic individuals conserve heat better in cooler areas. However, our finding that at least one ectothermic group also follows Bergmann's rule suggests that additional factors may be important. Several alternative processes, such as selection for rapid heat gain in cooler areas, may be responsible for the converse to Bergmann's rule in squamates.