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The western diamondback rattlesnake (Crotalus atrox) is sexually dimorphic (adult males larger than females) throughout its range, A comprehensive analysis of the ecological and evolutionary sources of sex differences in growth and size requires a detailed understanding of growth trajectories during ontogeny, We studied a pop ulation of C, atrox in the upper Sonoran Desert of central Arizona. Specifically, we asked at what point during ontogeny males and females diverge in size: at birth, during juvenile growth, or as mature adults? We used mark-recapture data and a novel analysis of rattle characteristics to test for sex differences during each of these three ontogenetic stages, Our analyses suggest that male and female neonates are similar in size. Juvenile growth rates also appear to be uniform between sexes. However, males and females begin to diverge in size beyond sexual maturity, when female but not male growth rates become undetectable given our recapture intervals. Our results indicate that, when partitioned into its Lifecycle components, sexual size dimorphism can be expressed at different stages and may be affected by a variety of behavioral, bioenergetic, and selective forces, Accordingly studies of proximate and ultimate causation in sexual size dimorphism may benefit from consideration of the ontogeny of size separately in males and females.