Data on temporal variations of total dimethylsulfoniopropionate (DMSPt) and the environmental factors that influence DMSPt concentrations are important in understanding the biogeochemical cycling of organic sulfur compounds. Annual and diurnal variations of DMSPt were investigated in relation to environmental variables at a fixed station in Dona Paula bay (west coast of India). DMSP concentrations were high in the day and low at night and ranged from 3.69 to 84 nM with a maximum at 17.00 h. The high concentrations of DMSPt during daytime closely followed that of Chl a concentrations. The DMSP utilizers averaged 0.8 plus or minus 0.3 x 10sup(3) cells l sup(-1) during night and 0.4 plus or minus 0.1 x 10 sup(3) cells l sup(-1) during the day. The diel variation of DMSPt was influenced more by biological variables than hydrographic parameters. In the year-round study, the concentrations ranged from 0.69 to 15.8 nM. It was fourfold higher during the southwest monsoon season (13.4 plus or minus 2 nM) and threefold higher during the post-monsoon season (9.96 plus or minus 5 nM) compared to the pre-monsoon season (3.1 plus or minus 1 nM). DMSPt concentrations showed temporal variability, both during diurnal and annual studies. Diatoms were identified as producers of DMSP in Dona Paula bay. Dinoflagellates also contributed during the non-monsoon seasons. Another factor involved in the variability of DMSPt was DMSP utilizing bacteria, which ranged from 1 to 10% of the total heterotrophic count