The effect of removal of the parietal eye (parietalectomy) was studied in free-living lava lizards at the Charles Darwin Research Station in the Galapagos Islands, and the results were compared with those obtained with parietalectomized temperate zone iguanids. Behavior, activity, body temperature, and Iodine-131 uptake by the thyroid gland were studied from Jan. 23 to Feb. 27, 1964. Parietalectomy had no detectable effect on any of the parameters investigated. Home ranges of adult male and female lizards were measured, and defensive and antagonistic behavior was noted. The home ranbges of males often encompassed those of several females. Most adults arose and retired with the sun, and sharply reduced exposure to sunlight at midday. At night, lizards buried themselves in soil and leaf-litter, and the same bed sites were often regularly used. Individuals injected with Iodine-131 were located at night with a scintillation counter. Active lizards had body temperatures averaging 34 to 35°C (range 22.6 to 39.0°C), which showed a bi-modal distribution. Cloud cover was a significant factor in body temperature fluctuations. Sexual dimorphism, both in color and in size of adults, was marked. Females had red "cheek" patches and were smaller than males.Most males had sperm in their sperm ducts and some females contained large-shelled oviducal eggs. Mating males seized females with their jaws, and sometimes carried them some distance before attempting copulation. Posturing and perhaps the odor of refractory females discouraged males. The lizards were found to be insectivorous, but some plant material was also eaten. One small gecko (Phyllodactylus galapagoensis) was recovered from stomach contents.