Ecology of biofouling on the edible estuarine oysterCrassostrea madrasensis (Preston) has been investigated in the Ashtamudi Backwater of the southwest coast of India. Fouling was highly conspicuous throughout the year and dominant groups included barnacles, serpulids, bryozoans and modiolids. Intensity of fouling varied from 73 to 179 % on living oyster valves and 65 to 172 % on dead valves with respective annual averages of 118 and 127%. Substrate selection and settlement of the different groups were mostly opportunistic. Barnacles were the most dominant, living and dead ones collectively contributing to about 26 % of fouling on living valves and 32 % on dead valves. Serpulid fouling was 22 % both on living and dead oyster valves, bryozoans 15 and 12%, modiolids 11 and 12% and the miscellaneous groups formed 27 and 20 % respectively. Availability of free settling space and fouling in relation to substrate size were also investigated. Total fouling was very intense on oysters of 25–35 cm2 size group. Impact of biofouling on oysters and certain earlier studies on the topic are discussed.