_Variation in Size and Scutellation._--The largest male has a snout-vent length of 132 mm., the largest female, 114 mm., and the smallest juvenile, 34 mm. The number of dorsal granules at the midbody varies from 91 to 117 (106.2 ± 0.43); the ratio of the number of granules between the paravertebral stripes to the number of granules around the body (PV/GAB) varies from 0.064 to 0.157 (0.097 ±0.007); the number of femoral pores varies from 32 to 49 (41.1 ± 0.20). Usually there are only three enlarged preanals, but 18 specimens have a somewhat enlarged scale anterior to the normal complement of three. In 15 specimens the supraorbital semicircle-series terminate short of the posterior edge of the frontal; in the others the series reach the frontal. _Variation in Coloration._--The coloration of juveniles and subadults varies little; large adults vary considerably especially in the amount of diffusion of the light green middorsal area. In some individuals the vertebral pale area does not include the paravertebral spots; in other individuals the pale area includes not only the paravertebral rows, but, at least anteriorly, the dorsolateral rows. In large males of about equal size (and collected at the same time) there is considerable variation in the amount of blue on the belly. In a few of the males the belly is white with only the anterior edge of each scale blue; in some only the lateral rows of ventrals on the posterior two-thirds of the body are blue; in others all of the posterior two-thirds of the belly is blue. _Ontogenetic Change in Color Pattern._--The metamorphosis of color pattern in _Cnemidophorus sacki zweifeli_ results in the dorsal ground-color becoming paler with age, the replacement of the stripes by spots, and finally in large males the suffusion of these spots. A single hatchling (UMMZ 114732) is available; this specimen has a prominent umbilical scar and a snout-vent length of 34 mm. The top of the head is olive brown; the dorsal surfaces of the limbs are dark brown with cream mottling; the dorsal ground-color is brownish black; this is paler on the lower flanks. The lateral and dorsolateral stripes are cream-colored; the paravertebral stripes are white. There is a faint, diffuse vertebral stripe anteriorly (Fig. 2 A). The throat and undersides of the limbs and tail are cream-colored; the belly is bluish white. In life the stripes were pale yellowish green, and the tip of the tail was pink. In larger individuals the dorsal ground-color is dark brown; the lower flanks are grayish tan. Light brown diffuse spots are present in the lateral and dorsolateral dark fields. The tan vertebral stripe is diffuse and nearly fills the paravertebral dark fields; the paravertebral stripes are faint posteriorly; throughout their length they are scalloped--the beginning of their fragmentation into spots (Fig. 2 B).