Benthic metabolism and the effects of bioturbation were investigated during the dry season from December to March in a tilapia (Oreochromis niloticus) dominated polyculture fish pond in northeast Thailand. Benthic metabolic fluxes were measured by incubating intact sediment cores sampled in the pond. Oxygen uptake showed significant temporal variations from 15.4 to 52.3 mmol O₂ m^-2 day^-1, averaging 31.9 ± 15.0 mmol O₂ m^-2 day^-1. Total carbon dioxide release showed similar temporal variations from 43.0 to 127.8 mmol CO₂ m^-2 day^-1, averaging 68.6 ± 36.9 mmol CO₂ m^-2 day^-1 (n = 6). The community respiratory quotient (RQ = CO₂/O₂; range 1.1-3.5, average 2.3 ± 0.9, n = 6) indicated significant anaerobic mineralisation. Oxygen uptake by the sediment corresponded to 8% of the oxygen respired by the planktonic community. Ammonia release averaged 13.2 ± 4.8 mmol NH₄/⁺ m^-2 day^-1 (n = 4). A new method using the stable ¹⁵N isotope technique for measuring denitrification was introduced. Rates of denitrification averaged 4.1 ± 0.85 mmol N m^-2 day^-1 (n = 3). Denitrification was insignificant in the mineralisation of organic matter, but caused atmospheric loss of nitrogen, averaging 16% of the loading by commercial fertiliser and manure. The net sedimentation of particulate organic matter in the pond was 5 cm year^-1. The burial of particulate organic carbon (POC) and particulate organic nitrogen (PON) averaged 25% and 60% of the gross input. respectively. The effects of fish and benthic fauna on the benthic metabolism were studied by excluding the fish from 0.5 m² of the pond bottom for 6 weeks. The sediment was dominated by small oligochaetes less than 10 mm long and the exclusion experiment indicated a high predation on the benthic fauna. Oligochaete numbers and biomass as well as benthic metabolic rates were significantly higher when fish were excluded from the sediment. Inside the exclosure, the RQ was approximately the double that outside, indicating that the exclusion of fish from the sediment increased the sediment flux of metabolites and the anaerobic respiration, including the denitrification process. The presence of benthivorous fish reduced the atmospheric nitrogen loss caused by denitrification by 50%.