Recent studies with cyprinid and characin (superorder Ostariophysi) fishes suggest that purine-N-oxides function as chemical alarm cues (alarm pheromones) and that the nitrogen oxide functional group acts as the chief molecular trigger. To further test the hypothesis that the nitrogen-oxide functional group is evolutionarily conserved as an active component of the Ostariophysan alarm pheromone system, we exposed juvenile channel catfish (Ictalurus punctatus, Siluriformes) to conspecific skin extract, hypoxanthine-3-N-oxide (the putative alarm pheromone) and a suite of structurally and functionally similar compounds. Conspecific skin extract and hypoxanthine-3-N-oxide elicited significant increases in species typical antipredator behaviors. A structurally dissimilar compound possessing a nitrogen oxide functional group (pyridine-N-oxide) elicited a significant, but less intense alarm response. Compounds lacking a nitrogen oxide functional group were not significantly different from control stimuli. In addition, two non-Ostariophysan species known to possess chemical alarm cues (convict cichlids, Acrchocentrus nigrofasciatus, Cichlidae, Acanthopterygii and rainbow trout, Oncorhynchus mykiss, Salmonidae, Protacanthopterygii) did not show any increase in antipredator behavior in response to hypoxanthine-3-N-oxide. These data demonstrate the conservation of chemical alarm cues within at least three orders of the superorder Ostariophysi.