To study possible intergradation between Aspidoscelis tigris munda (west) and A. t. tigris (east) through Walker Pass, a low-elevation corridor in the southern Sierra Nevada, California, I collected 90 individuals of A. tigris from 26 May through 2 June 1994 from a 24-km transect with 3 evenly spaced sampling sites representing populations on the summit (1,555 m), west slope (1,098 m), and east slope (1,014 m). The central (summit) population lacked phenotypic intermediacy in body size (larger), color (greater melanism), and multivariate morphological variation (means of first canonical variate: summit > west > east). In addition, egg clutches were present only in females from the summit population. Stepwise regression of snout-vent length (SVL) on 6 environmental variables revealed that body size was related positively only to winter precipitation. Dominant vegetation of the 3 sampling sites substantiates the presence of a pronounced rain shadow at Walker Pass. It is likely that the phenotypic and reproductive differences among the 3 populations are related to the greater amount of critical winter precipitation at the summit of the pass.