We studied the behavior of the Poeciliid fish Gambusia affinis after the introduction of 3 substances into their tank: a homogenization of Gambusia affinis, a homogenization of the Anabantid fish Betta splendens, and a blank made of distilled water. The response of the fish was measured as a change in their spatial distribution in the tank after the introduction of the substance. Two sizes of fish were used, and the Gambusia homogenization produced clear alarm reactions in both, the fish fleeing to the bottom of the tank. This is one of a few examples available of recognition of alarm substances in non-ostariophysian fish. In addition, we found that the small fishes that had recently been exposed to the alarm substance stayed in an 'alert state', in which they had an increased sensitivity to mechanical and visual fright stimuli.