Cnemidophorus costatus barrancorum was studied in the short-tree forest of southeastern Sonora, Mexico from November 1985 to December 1988. Adult females varied in snout-vent length from 71 to 101 mm, while males reached a maximum of 121mm. Ontogenetic change in color-pattern was pronounced, however sexual dimorphism in color-pattern was slight. The mean number of dorsal scales around mid-body was 99.4. Mating occurred in late July. Females laid two clutches of eggs per season, with clutch size varying from 2-8 (X = 4.2). The maximum shelled egg dimensions were 10.5mm x 15.6mm. Hatchlings appeared in early fall, and were surface-active throughout November. Yearlings emerged from hibernation in February-March. Adults were active from mid-spring into September. C. c. barrancorum had a unimodal daily activity pattern, with activity beginning when air temperature exceeded 30°C. The mean body temperature of surface-active animals was 40.0°C.