The bioconcentration of 2,4,5,2',4',5'-hexachlorobiphenyl (HCB) was examined in the Great Lakes algae Fragilaria crotonensis, Ankistrodesmus falcatus, and Microcystis sp. The bioconcentration factors varied from species to species, whether they were expressed in terms of cell number, dry weight, cellular carbon, or cellular lipid. The factors were in the range of 100,000-1,000,000 and increased with decreasing biomass. The existence of a mucilage layer in F. crotonensis was associated with a twofold increase in the bioconcentration factor. Surface adsorption appeared to contribute only slightly to the bioaccumulation of HCB. HCB desorbed from all species but at a much slower rate than its adsorption.