The social behavior patterns of both natural and enclosed populations of Sceloporus undulatus and Cnemidophorus sexlineatus were observed in Oklahoma. During challenge display, the male Sceloporus compresses his trunk, extends his dewlap, orients laterally towards his opponent and performs push-ups. The push-up series follows a speciesspecific sequence of units of movement: double, single, followed by a series of doubles, performed to a specific cadence in time. The dominant male is the most active, challenging and chasing other males, but tolerating females. Subordinate males retreat or assume a submissive posture. Female aggression is indicated by postural changes accompanied by sidlehopping. Sex recognition depends upon the response to a challenge. A courting male nods rapidly while following a female. During copulation, the male maintains a biting hold on the shoulder of the female. A dominant male readily challenges his image in a mirror. Under crowded conditions of an enclosure dominance may be indicated by one individual supplanting another at a favorable lookout. Sexual arousal in the male Cnemidophorus is indicated by cloacal rubbing. During courtship the male follows the female and may walk astraddle her. During copulation, the male maintains a biting hold on the flank of the female as his body arches across her. The dominant males aggressively chase all others and will attempt to mate with submissive males. The behavior patterns of these two lizards show differences relative to adaptation to their respective habitat niches. Each species of animal in a biotic community plays a certain ecological role in the community as a whole. The best understanding and clearest insight into its ecological niche can, perhaps, be obtained through comparison with its closest competitor. In Oklahoma, two species of lizards that occupy the same habitat are the fence lizard (Sceloporus undulatus) and the six-lined racerunner (Cnemidophorus sexlineatus). Through the simultaneous observation of both species, important qualities of difference in ecology and behavior can be more easily recognized and their adaptive significance within the community established. The six-lined racerunner (Cnemidophorus sexlineatus, Teiidae) is found throughout Oklahoma. Adults may reach a body length of 8 cm with a total length of 24 cm. Only slight sexual dimorphism is apparent, where the stripes of the male are partially obliterated by the suffusion of green, and the belly takes on a bluish tinge in contrast to the prominent stripes and white belly of the female. Sceloporus undulatus (Jguanidae) has four described races within Oklahoma; S. u. hyacinthinus, the northern fence lizard, S. u. consobrinus, the southern prairie lizard, S. u. garmanti, northern prairie lizard, and S. u. erythrocheilus, the northern plateau lizard. These races vary in adult size (snout-vent length) from two inches to a little under three inches; garmani being the smaller and erythrocheilus the larger of the four.