Algae were sampled from unionid shells and unconsolidated sediments near ( 0.05 m) and far ( 1 m) from unionids, in two distinct habitats ( 8 m deep silty areas and  1 m deep sandy areas) in Douglas Lake, Michigan. Epizoic communities (dominated by Batrachospermum sp., Oscillatoria spp., and Lyngbya sp.) differed markedly from sediment algal communities. Algal communities in the deep, silty habitat differed from those in shallow sandy habitat; Oscillatoria spp. were more common in silt whereas several diatom genera were common in sand. Proximity to unionids did not greatly alter sediment algal community composition. Experimental comparison of algal communities grown on live unionids and non-living shells suggested that a live unionid host increased the relative abundance of Lyngbya sp. but not other common epizoic filaments (e.g., Batrachospermum sp., Oscillatoria spp.).