Vertical migration and feeding periodicity of daphnids were monitored for three summers in three lakes, two of which had undergone reciprocal fish transfers during May of the second year. In Peter and Paul Lakes, the diel migratory behavior of the Daphnia assemblages was fairly constant. In Tuesday Lake, where fish manipulations greatly reduced planktivory. Daphnia pulex became abundant and exhibited high variability in the direction of diel migration. Daphnids of Tuesday Lake were smaller and more recently established than those of the other lakes. Weather and lunar phase did not correlate well with migratory or feeding behavior, though there was a negative correlation with the chlorophyll a content of the mixed layer. In this study high planktivory by fish was associated with pronounced and consistent nocturnal migration. Low planktivory by fish was associated with variable, inconsistent migration. Evidently, planktivorous fish are essential for maintenance of vertical migration in these daphnid populations.