Diaptomus sanguineus, a small freshwater copepod, avoids periods of intense fish predation by producing diapausing eggs. We developed a computer simulation of the copepod's life history and used it to compete populations that switched to diapause at different intervals of time before the onset of fish predation (the catastrophe). With no variation about the catastrophe date, the evolutionarily stable strategy (ESS) is one in which the switch to diapause comes exactly one generation before the catastrophe, as Taylor (1980) has shown analytically. With increasing variation about the catastrophe date, the ESS becomes one of switching to diapause at time intervals increasingly greater than one generation. Using field data on copepod mortality rates from 5 yr, we have estimated the mean and variance about the catastrophe date. Using field and laboratory data, we have estimated the copepod generation time and the timing of the switch to diapause. We find, in close agreement with simulation-derived ESSs, that D. sanguineus starts making diapausing eggs 1.3 generations before the major onset of fish-induced mortality.