The rates of cellulose breakdown, composition of detrital microflora, and density of bacterial populations were determined in the epilimnetic sediments and water columns of two poorly buffered, oligotrophic, Canadian Shield lakes having mean surficial pHs of 4.6 (Bat Lake) and 6.6 (Harp Lake). The decomposition rate was significantly lower in oxic sediment of the acidified lake than of the circumneutral lake, but water column rates were almost identical in the two lakes. These results are explained in terms of the groups of cellulolytic microorganisms which were observed by phase-contrast microscopy as being active at the different sites: fungi in Bat Lake water and Cytophaga-like bacteria in the water and sediment of Harp Lake. Cytophaga-like bacteria were also the main decomposers in Bat Lake sediment, but their activity was restricted at porewater pHs of <5.0. Acridine orange direct counts of bacteria in the top centimeter of sediment ranged from 3.7 × 108 to 1.0 × 109 per g, and counts in planktonic water samples ranged from 4.9 × 105 to 1.2 × 106 per ml. Bacterial densities at most sites decreased significantly (P < 0.001) from August to late October, but did not show a consistent pattern of differences related to pH.