Microbial biomass and community structures were determined in sediments lining the burrows of 3 species of marine worms, and in nearby surface and subsurface sediments, using esterlinked phospholipid fatty acid (PLFA) analysis. The potential impact of biogenic bromophenols produced by 2 of these animals on burrow microbial communities was of particular interest. The burrow microbial communities were markedly different from those of surrounding surface and subsurface sedi m e n t~. Differences in microbial biomass were attributed to burrow structures, textures of the burrow lining sediments, and organic carbon content. No significant reduction of microbial biomass or of several distinctive signature PLFA was detected in bromophenol-contaminated burrows, -when compared to non-bromophenol-containing burrows. All 3 types of sediments (burrow, surface, and subsurface) examined for each worm species were distinct as determined by multivariate cluster analysis of PLFA profiles. Signature lipid biomarker PLFA for Gram-negative bacteria, Gram-positive bacteria, sulfatereducing bacteria and anaerobes, and microeucaryotes were readily identified in burrow sediments from bromophenol producing and non-producing worms.