Big-bellied seahorses, Hippocampus abdominalis (Chordata: Syngnathidae), feed predominantly on swarming mysids in southern Tasmania. We tested the possibility that kairomones mediate this predator/prey interaction. ‘Fish water’ was prepared by holding one seahorse in 4 l of seawater for 1 h and using this water within 1 h to test for presence of kairomones. One ml of this water pipetted into a tank containing five mysids, Paramesopodopsis rufa (Arthropoda: Mysidacea), induced a significantly increased number of tailflips (mysid escape response) compared with control seawater. The same effect was seen whether seahorses were fed or starved immediately before the experiment. This effect was not seen when realistic concentrations of excretory products, either ammonium hydroxide or urea, were used instead of fish water. When seahorses were kept in visual contact with mysid prey, but unable to capture them, subsequent testing of the ‘fish water’ in the same way as above did not produce a significant increase of tailflipping in mysids. Thus it appears that, when attacking, seahorses can suppress release of kairomones in order to remain chemically inconspicuous to their prey. This is the first demonstration of this phenomenon. When mysids in a cohesive swarm (65 or 100 individuals) were exposed to ‘fish water’, no significant anti-predator response i.e. decrease in swarm volume, could be detected. We interpret these results to indicate the greater vulnerability of mysids when not in social groupings (swarm or school) and the higher likelihood of an energetic response (particularly tailflipping) to a threat.