Denitrification in the oxygen-poor waters at intermediate depths in the Arabian Sea is very intense. Depth profiles of nitrite and the activities of the electron transport system (ETS) and dissimilatory enzymes such as nitrate reductase and nitrite reductase within the nitrite-bearing waters, display depth-mismatched maxima. There are indications of contribution by both dissimilatory nitrate reduction and nitrification to the nitrite enrichment of the secondary nitrite maximum (SNM) between 150 and 350 m. The waters below the SNM, not bearing nitrite but within the oxygen minimum zone, are also potentially denitrifying. Analyses of the relationships between various parameters indicate the existence of a layered ecosystem in which nitrification and denitrification activities are present in several closely-spaced layers. This is probably caused by the inflow of different water masses into this hypoxic region, as suggested by the strong maxima and minima in enzyme activities at equivalent depths.