Lake Aydat is a young eutrophic lake (mean depth 7.4 m, area 6o ha) situated in the french Central Massif. Secondary production was studied during the summer months June and July. In early summer, dissolved oxygen in the deep layers gradually gets depleted, and depletion reaches the 7m level in July. Zooplankton is therefore concentrated in the upper layers: Rotatoria are very abundant and the various species are segregated in space and / or succeed each other in time, thus maximally exploiting the water layers favorable to their development. Cladocera are represented by small species, which sharply increase in numbers at the time of maximum primary production, and cause a dramatic drop in primary production. They are in competition with the Rotatoria. Copepods are the least abundant group, but they were in a reproductive phase at the time of this study.