Intensive observations in the northeast Indian Ocean (Bay of Bengal) during the pre-southwest and northeast monsoon seasons 1991 reveal that freshwater discharge from rivers of the Indian subcontinent exerts the dominant control over total carbon dioxide (TCO sub(2)) and pCO sub(2) distributions in surface waters. Low pCO sub(2) levels occur within the low-salinity zones, with a large area in the northwestern bay acting as a sink for atmospheric CO sub(2) . Only a part of the observed pCO sub(2) variation can be accounted for by the effect of salinity, and biological production supported by external nutrient input in conjunction with strong thermohaline stratification may be more important in lowering surface water pCo sub(2) by greater than100 mu atm relative to that in the atmosphere. The pCO@2) distribution is seasonally variable and appears to be controlled by the spreading of fresh waters by the prevailing surface circulation