The responses of crayfish to water from aquaria containing either undisturbed or disturbed animals were observed. The crayfish Orconectes propinquus and O. rusticus showed no response to disturbed-conspecific water. Individuals of O. virilis respond not only to disturbed crayfish but to other taxa (the leech Macrobdella decora, the darter Etheostoma exile, and rock bqass Ambloplites rupestris), but not to the painted turtle Chrsymes picta. Additional tests indicated partial responses by O. virilis to ammonium and to a chemical or chemicals released from the green gland of crayfish. Ablation experiments indicated the antennules as the site of reception of the chemicals. Additional behavioral tests indicated that detection of the disturbance chemical(s) results in the crayfish showing low-level alert for more than 15 min, once an initial priming period has passed.