The predatory strikes of a sidewinder rattlesnake (Crotalus cerastes) can be avoided by kangaroo rats (Dipodomys merriami) in the normal condition and also after either of two surgical manipulations: (1) removal of the eyes, or (2) reduction of middle ear volume. When both these operations are performed, however, the kangaroo rat is struck by the rattlesnake with relative ease.Reduction of middle ear volume alone in a natural population of kangaroo rats resulted in a high loss of animals with reduced middle ear volume; most of them disappeared from the population during the dark phase of the moon. It is concluded that both vision and audition play significant adaptive roles in predator avoidance.