Morphometric measurements of Daphnia carinata from both a pond and a lake in Canterbury, New Zealand, showed seasonal changes in tail spine length occurred in both populations of Daphnia. In the pond form, a marked dorso-anterior extension of the carapace (crest development) also occurred. Morphological changes in pond Daphnia were associated with an increase in both density and size of the predatory backswimmer Anisops wakefieldi, while the increase in tail spine length of lake Daphnia was associated with increases in both temperature and mite (Piona exigua) density. Laboratory and field enclosure experiments showed that Anisops presence can cause the observed morphological changes in pond Daphnia. Pond Daphnia that had developed crests and long tail spines in the presence of Anisops showed either an increase or else no difference in fecundity compared with uncrested individuals with short tail spines that occurred in the absence of Anisops. There was no evidence of a life history cost associated with the morphological change.