Using a clone that responds to the presence of fishkairomones by a pronounced change in phototacticbehaviour, we determined how fast a change to morenegatively phototactic behaviour occurs in Daphnia magnaadults that are exposed to a highconcentration of fish kairomones. Kairomone exposedanimals showed an approximately linear decrease in thevalue of the phototactic index with time. Though theresponse was almost immediate, it took two hoursbefore the difference between fish-induced and controlanimals was significant. Extrapolation of the observedresponse indicates that a maximal change inphototactic behaviour, equivalent to animals that havebeen cultured in the presence of fish kairomones sincebirth, occurs after about 13 hours exposure. Weconclude that the predator-induced change in dielvertical migration of zooplankton is fast, and isfully developed in less than a day. The response timeto fish kairomones of Daphnia is shorter forphototactic behaviour than for life history traits,which may have important consequences with respect tothe evolution of trait-dependence in induced defenceresponses.