SUMMARY. The anaerobic decomposition of particulate organic matter (POM) was examined in the anoxic pelagic sediments of hypereutrophic Wintergreen Lake. Degradation of sedimented POM occurred rapidly as shown by increased production and release of ammonia, hydrogen sulphide, volatile fatty acids and methane from the sediments 2–3 weeks after large inputs of organic matter. Maximum concentrations of most metabolites were found at the sediment-water interface, indicating that the initial anaerobic degradation of freshly deposited POM occurred at this site. The absence of the inorganic electron acceptors, nitrate and sulphate, suggested that fermentation and methanogenesis were the major anaerobic processes involved in the dissimilation of organic matter in these sediments during stratified periods. The amount of carbon input converted to methane in the sediments was determined from May to early November 1976 and 1977. Carbon output as methane was measured by quantifying methane lost from the sediments by ebullition and by estimating soluble methane lost to the water column by diffusion. Total methane release during summer stratification accounted for 34% of the particulate organic carbon input to the sediments in 1976 and 44% in 1977. Methane release was directly related to the rate of sedimentation of POM. However, methane production was temporarily inhibited following high rates of sedimentation in 1976, suggesting that the rate of organic loading may be an important factor controlling anaerobic decomposition in these sediments.