Particulate organo-Se was assimilated with 86% efficiency by the deposit feeding bivalve Macoma balthica, when the clam was fed 75 Se-labeled diatoms. Absorption efficiencies of particulate elemental Se were 22%, when the animals were fed 75 Se-labeled sediments in which elemental Se was precipitated by microbial dissimilatory reduction. Precipitation of elemental Se did not eliminate biological availability of the element. Selenite was taken up from solution slowly by M. balthica (mean concentration factor was 712)