At the proposed site for the Ocean Thermal Energy Conversion power plant on the east coast of India, biolm formation and its control using intermittent chlorination in plate heat exchangers was studied prior to commissioning of the plant. Settlement of fouling larvae was not observed on the plates, which may be attributed to the low density of larvae of fouling organisms in the open ocean (65 km from the coast). Signicant reduction in biolm thickness was observed after chlorination at a dosage of 1:2 ppm residual. Microalgae were absent on untreated and chlorinated heat exchanger plates. Signicant di6erences were also observed with respect to bacterial density and diversity between chlorinated and control surfaces. Total counts of viable bacteria in untreated controls increased with time. Relative to controls, a signicant reduction in bacterial density in biolms was observed upon chlorination. Sulphate reducing bacteria were absent in chlorinated biolms, whereas their numbers increased with time in controls. Counts of heterotrophic bacteria in biolms also showed an increase with time in controls and were signicantly lower on chlorinated surfaces. Counts of slime formers, e.g. Pseudomonas sp. and Aeromonas sp., were low compared to total viable bacterial counts. ? 2003 Elsevier Ltd. All rights reserved.