An unexpected increase of phototrophic and heterotrophic pico-and nanoplankton under anoxic conditions was encountered in the central Baltic Sea below 120 m depth in August 1991 and 135 m depth in July 1992, respectively. Cell abundances in anoxic layers were up to 10 times higher for chroococcoid cyanobacteria, 6 to 8 times for phototrophic nanoflagellates and 3 to 4 times for bacteria, as compared to the overlaying aphotic oxic layers. A characteristic ciliate community occurred at the oxic/anoxic interface. Phototrophic organisms of anoxic depths were capable of productivity when reoxygenated and returned into the light. Considerable grazing on bacteria (0.41 to 0.65 d ' ) and chroococcoid cyanobacteria (1.19 to 1.29d-') was revealed by serial dilution experiments. Cyanobacteria showed gross growth rates of 0.85 to 1.33 d-' under aphotic/anoxic conditions. The origin of deep-water phototrophic pico-and nanoplankton and the existence of a permanent anaerobic microbial food web are discussed.