For animals that use crypsis to avoid predators, immobility reduces the risk of detection. The magnitude of this immobility benefit depends upon the probability that a predator is present, since a predator must be present for crypsis to be valuable. Thus, cryptic animals typically reduce their movement rates upon detection of a nearby predator or signs of its activity. Such a response occurs in tidepool sculpins (Oligocottus maculosus) when presented with water-borne compounds released from the skin of injured conspecifics (Hugie et al. 1991). The benefit of immobility should also depend upon the animal's background, or substrate, since animals on a matching substrate achieve a higher level of crypticity than those on a nonmatching substrate, and have more to gain by remaining still. Therefore, we predicted that the response of tidepool sculpins to conspecific skin extract would involve a greater reduction in movement rates for fish on sand (matching) than for those on white (nonmatching) substrate. The results of a laboratory experiment supported this prediction, with fish on sand showing a large decrease in movement rates in response to skin extract, while the movement rates of those on white substrate remained unchanged.