Samples comprising 51, 52, and 24 specimens of Cnemidophorus bacatus, the endemic whiptail lizard of Isla San Pedro Nolasco, Gulf of California, were obtained in August 1964-1967. This species has been only briefly mentioned in the literature since it was originally described. Analysis of the new material supported the view that C. bacatus is one of the smallest species of the TIGRIS species group. This species is recognizable by a combination of 4-4 supraocular scales, granular postantebrachial scales, small mesoptychial scales, approximately 74 granules around midbody, 164 granules from the interparietal scale to the base of the tail, 37 femoral pores combined, 30 subdigital lamellae, 13 circumorbital scales, 13 lateral supraocular granules, and small body size. Juveniles have a blackish brown dorsum with a pattern of stripes and spots, and a slate black venter; whereas adult males and females have a dark brown ground color with a pattern of spots, and a black venter. The sex ratio of 124 sexable C. bacatus was 649 9 :60 8 8. The presence of recent hatchlings and gravid females in the population at the same time may indicate that C. bacatus produces a minimum of two clutches per year. Clutch size varies from one to three, average near two. Mean clutch size on the basis of yolked ovarian follicles was 1.89 (72/38); on the basis of oviducal eggs and corpora lutea, 1.70 (17/10); and on the basis of a combination of these methods, 1.85 (89/48).