Individuals of two species of crayfish (Orconectes virilis and O. propinquus) were tested in the laboratory for responses to chemicals released from physically damaged conspecifics. Individuals of O. propinquus did not show an alarm response to crushed conspecifics. Individuals of O. virilis responded to a water-borne substance released from crushed conspecifics by assuming an intermediate posture and ceasing movement. Similar alarm responses were shown by individuals of O. virilis to crushed congeneric individuals (O. propinquus), and these responses were not eliminated by either freeze-thawing the crayfish used to prepare the signal or by treating freshly crushed crayfish with the enzyme trypsin. Individuals of O. virilis showed strong feeding responses to solutions prepared from frozen fish flesh but showed a mixture of alarm and feeding responses to freshly killed fish. These results indicate that the alarm substance used by O. virilis is widespread.