During RV Gaveshani cruise No. 235 (August-September, 1993), particulate matter was collected from euphotic and deeper waters in the Bay of Bengal and analysed for biogenic silica (BSi), lithogenic silica (LSi) and particulate organic carbon (POC). Mean depth profiles of BSi showed a minimum in the surface waters and a maximum in the middle depths of the euphotic zone. The deepening of BSi maxima from north to south coincides with the deepening of the thermocline and thickening of the surface layer. In coastal waters, the contribution of BSi to total particulate silica was high (> 80 %); in offshore waters it was 60-70 %. BSi/POC (0.086) and LSi/POC (0.023) ratios show that particulate matter consists mostly of biogenic material, while the BSi/LSi (1.61) ratio revealed the dominance of siliceous phytoplankton in the euphotic zone. High concentrations of BSi (0.49 mu mol l(-1)) and LSi (1.89 mu mol l(-1)) in deeper waters are due to a high sedimentation rate, the resuspension of bottom sediments and the flow of large amounts of riverine siliceous material.