We placed 72 plasticine replicas of large and small snakes near a California ground squirrel (Spermophilus beecheyi) colony in San Diego County, California. Ground squirrels aggressively confronted both large and small replica snakes. Ground squirrels were more aggressive when attacking small replicas relative to large. Ground squirrels bit the smaller replicas a greater number of times, concentrating their attacks near the head of the replica. Large replicas were attacked more frequently near the tail. From the results we conclude: 1) California ground squirrels recognize increased risk of attacking larger versus smaller snakes, and 2) use of plasticine replicas is an effective method to investigate antipredator behavior of prey.