In the spring of 1992 a surface bloom of Chrysochromulina spp. occurred in Danish waters. Vertical community structure was studied at six stations situated in the centre of the bloom. The biomass of grazers accounted for an average of 15% of the phytoplankton biomass and was dominated by heterotrophic dinoflagellates and copepods. Ciliates were almost excluded from the plankton. In spite of the high grazer biomass, loss of phytoplankton biomass due to grazing was low. No mortality was observed in the mesozooplankton during incubation with bloom water. However, about 50 tons of dead rainbow trout,Oncorhynchus mykiss were observed in association with the bloom.