The structure of bacterial communities, the distribution of sulfide and oxygen, bacteriochlorophyll concentrations, and the temperature profile were determined for the anaerobic hypolimnia of two lakes in southern Michigan. Information from these studies, plus qualitative observations of two other lakes and two ponds over a 4-year period were used to correlate the spatial distribution of the populations, cell size, arrangement of photosynthetic vesicles or lamellae, presence of gas vacuoles or flagella, sulfur deposition, and environmental factors. On the basis of these results, three communities designated as A, B, and C were defined. The upper (A) community consisted of sequentially layered purple sulfur bacteria including two or more of the following genera: Thiopedia, Thiospirillum, Thiocystis, or Chromatium. The middle (B) community consisted of sequential layers of green bacteria from one or more of the following genera: Pelodictyon, Clathrochloris, Chlorochromatium, or Prosthecochloris. The lowest (C)community contained previously unreported gas-vacuolate colorless bacteria 0 to 0.7 m above the sediment. Microstratification (0.1- to 0.2-m layers) of populations was observed within the A and B communities.