The vertical distribution of porewater chemistry, and the numbers and activities of methane‐producing bacteria (MPB) and sulfate‐re‐ducing bacteria (SRB) were examined in shallow subtidal marine sediment cores. Sulfate concentrations decreased sharply below 5 cm in three of the four cores collected, and methane levels were highest (2500 to 3500 nmol/ml) immediately below the depth where sulfate was depleted. The fourth core (BM‐2) showed little sulfate depletion with depth and contained low levels of methane (less than 15 nmol/ml). Upon extended incubation at 20°C, rates of methano‐genesis were highest immediately below the high sulfate zone and decreased with depth. Sulfate reduction rates were highest in surface layers and decreased sharply with depth. The respiratory index [Rl =14CO2/(14CO2=14CH4)] for [2‐14C] acetate metabolism was high in the presence of high levels of sulfate and decreased sharply in the core sections low in sulfate. In the high sulfate BM‐2 core, meth‐anogenesis was not detected and an Rl of 1.0 was calculated at all depths where radioactivity was detected. Bacterial counts revealed that populations of MPB were 1 to 2 orders of magnitude less than SRB at all depths. Numbers did not decrease sharply below surface sediments, but the activities of these groups did decrease. These results demonstrated that sulfate reduction dominated over methane production in regions of high sulfate, while the relative importance of methanogenesis increased in low sulfate regions. The magnitude of the rates of either process, however, was more related to depth in the sediment core than to sulfate concentration.