Clones were isolated from natural populations with contrasting fish predation pressure and were tested for their phototactic behavior in the presence and absence of fish and/or fish-mediated chemicals. The presence of fish-mediated chemicals induced a more negatively phototactic behavior in all clones tested. This response was observed within 24 h after exposure to these chemicals. Release from the presence of the fish-mediated chemicals resulted for most clones in a gradual change back to control behavior, which took several days. There was no indication of a maternal effect. The response was not enhanced by a more direct exposure to the presence of fish, suggesting that the stimulus is a chemical.