Thraustochytrid protists and fungi were isolated and enumerated in culture from detritus of the brown alga Sargassum cinereum. Both groups occurred epi- and endobiontically in the detritus. The thraustochytrid Labyrinthuloides minuta occurred in healthy as well as decomposing algal tissues, whereas Ulkenia visurgensis was found only in the latter. Populations of all 3 groups (thraustochytrids, fungi and bacteria) were lowest in healthy algae containing higher amounts of phenolics than in detritus. Concentrations of total carbohydrates (TCH), total phenols, proteins, alginate and mannitol, as well as C/N ratios declined as decomposition progressed, concomitant with the increase in microbial biomass. Both thraustochytrids and bacteria attained peak biomass values in 21 d detritus. Maximum observed biomass of thraustochytrids corresponded to 0.07% C and that of bacteria to 1.1% C of detrital dry weight