On May 14, 1946 at 8 o'clock in the morning the writer, driving southward, was nearing Hermosilla, Sonora, Mexico. Observing a Gila monster (Heloderma suspectum) to crawl across the road ahead of the car, a stop was made. The writer easily overtook the big lizard as it lumbered along in a determined way, deviating just enough to miss large stones and other obstructions. On being approached closely, the animal immediately stopped his forward progress and assumed a defiant attitude with legs braced, head elevated and mouth open. Several violent hisses were given as it snapped at the stick with which numerous attempts were made to pin the animal on its back. Its writhing demonstrated strength and persistence. It appeared to have better coordination and greater agility in the fight than in the previous flight. The statement (Woodson, 1944) that the species is aggressive in nature and docile in captivity was, at least, half true. A quick survey of the animal's environment disclosed a relatively level terrain sparsely covered with chaparral and scrubby trees characteristic of the Upper Sonoran Life Zone. No human habitation or other indication of open water was in sight. Where did the Gila monster drink? When would it get its next meal? How often must it take water and food? If the eggs of birds are the principal item of diet, can this lizard endure the fast of some eight or ten months between the breeding seasons of birds? But little is known of the habits of this animal and fabulous tales comparable to those of many kinds of snakes (Schmidt, 1929) are frequently heard about the Gila monster. Therefore, the opportunity seemed at hand for observing and learning certain truths about the Gila monster. With due respect for the venomous nature of the Gila monster (Woodson, 1947), an improvised cage was made from a gallon oil can and a bit of screen on the following day. The animal was transferred to it and kept without food or water until the writer returned on May 23. On arriving in Los Angeles, after its fast of nine days it was assumed that the animal was hungry, and a live, one-fourth grown white rat was placed in the cage. The rat showed no fear of the Gila monster and frequently climbed on the lizard's back. The Gila monster was alert to the movements of the rat, followed it about the cage and repeatedly thrust out its tongue and touched it. The following day (May 24) a hen's fresh egg was placed in the cage, which egg the lizard ate almost at once. Three days later the rat was removed, as the lizard had not been observed to make any attempt to attack it (Hensley, 1949). Although eggs were fed for months thereafter, several other types of food