Using a clone that responds to the presence of fish kairomones by a pronounced change in phototactic behaviour, we determined how fast a change to more negatively phototactic behaviour occurs in Daphnia magna adults that are exposed to a high concentration of fish kairomones. Kairomone exposed animals showed an approximately linear decrease in the value of the phototactic index with time. Though the response was almost immediate, it took two hours before the difference between fish-induced and control animals was significant. Extrapolation of the observed response indicates that a maximal change in phototactic behaviour, equivalent to animals that have been cultured in the presence of fish kairomones since birth, occurs after about 13 hours exposure. We conclude that the predator-induced change in diel vertical migration of zooplankton is fast, and is fully developed in less than a day. The response time to fish kairomones of Daphnia is shorter for phototactic behaviour than for life history traits, which may have important consequences with respect to the evolution of trait-dependence in induced defence responses.