The latency of food retrieval and other behaviours in the presence of a live boa constrictor and other snake-like stimuli was observed in three groups of squirrel monkeys (adult feral, adult laboratory reared, and immature laboratory reared). The principal results were as follows: 1. Both of the laboratory reared groups retrieved food within about five or fewer sec in the presence of any stimulus, including the live snake. 2. The retrieval latencies of the adult feral group were longest in the presence of the live snake, the next longest in the presence of rubber snakes painted either gray or an approximation of the boa constrictor markings, and shortest in the presence of various rubber tubes or a gray block. This result suggests that avoidance of some snake like stimuli in the adult feral squirrel monkey is determined more by shape than by markings. 3. The habituation of the snake avoidance response was rapid for all stimuli, but occurred more slowly for the live snake than for the two rubber snakes. 4. Of the various behaviors observed in the interval between the presentation and the retrieval of food, visual scanning was predominant and occurred during 74 percent of this time.