The biomass and structure of microbial communities colonizing barite freshly deposited onto natural marine mud was investigated in a laboratory flume. Modern biochemical methods based on the lipid content of microbial cells were used to quantify microbial colonization. The experiment involved the following 3 treatments: (1) fresh barite deposited onto the surface of a natural muddy sediment; (2) fresh silica (utilized as a colonization control) also deposited onto natural mud; and (3) the muddy sediment itself. The treatments were placed in an open-channel flow with 10 pm filtered sea water and sampled every other day for 16 d. The flow was set at a shear velocity (u.) of 0.30 cm S-', less than the critical U. to initiate sediment motion of these particles. After 48 h, the barite and silica treatments contained approximately 60 times less microbial biomass than the natural mud. Little change in microbial biomass was observed in the barite and silica treatments until 13 d of incubation when microbial blomass began to increase. The carbon content of the fresh barite and silica treatments was 60 to 570 times less than in the natural mud and this lower carbon content is the likely proximal cause of the lower microbial biornass. The structure of microbial communities was analyzed after 13 d of incubation. Despite the large difference in total biomass, microbial communities in the natural mud and barite treatments were similar in structure. In the silica treatment, however, community structure was highly variable and dissimilar to the natural mud community. The community in flowing sea water was more similar to communities in natural mud and barite than in silica. These results suggest that natural marine sediments receiving inputs of barite can be expected, given sufficient time, to develop microbial communities similar in structure and possibly biomass to those found in ambient sediments.