Under seasonally fluctuating redox conditions in sediment of Kiel Bay (eastern Baltic Sea), viable counts (MPN) of sulfate-reducing bacteria (SRB) ranged between 4 x 102 and 7 x 104 cm-3. These MPN appeared fairly independent of ambient redox potentials and followed peaks of phytoplankton productivity in the water column with a time lag of 2 to 3 wk. Shells of the clam Arctica islandica, which can survive anoxia, were, even in oxic sediments, colonized by epizoic SRB. It is hypothesized that SRB associated with benthic infauna represent the biogeochemically more reactive group. -from Authors