Desmarestia viridis,known to have an extremely low pH cell sap, supported rapidly increasing yeast populations in aquatic model systems. Population levels reached 108 yeasts ml-1 with an approximate doubling time of 1d. During incubation,pH declined in all systems except those receiving added carbon sources. Maximum yeast populations were not influenced by the initial experimental acidification of the medium or the presence of antibiotics added to suppress the growth of bacteria. In an unsupplemented system, Rhodotorula rubra and R. glutinis were the dominant yeasts. Systems with added organic material were dominated by a non-pigmented yeast. Examination of the algal filaments after 11 wk demonstrated little difference between experimental treatments, and that the filaments retained their original shape. Measurement of organic content in aquatic systems indicated little remaining soluble material. Similar population trends (i.e. yeast dominance and reduced bacterial populations) were observed in sand microcosms. Under these conditions, the algal filaments lost their original shape and were converted to a detritus-like substance.