The paper presents some of the important differences in the oceanographical and biological conditions in the waters of the Arabian Sea and the Bay of Bengal along the Indian coasts. In discussing the biological variability of the two environments from the point of view of fishery productivity, attention is particularly drawn to the extremely complex nature of the oil sardine and mackerel fisheries, especially along the west coast of India, which accounts for the bulk of the fish landings. An attempt is being made to provide a suitable explanation for this on the basis of the nutrient distribution pattern associated with the occurrence of seasonal ‘upwelling’. It is also pointed out that the ‘upwelling’ processes along the west coast of India are also of a complex nature and the forces leading to this are yet to be properly understood. Some comparison is drawn between the conditions on the south-west coast of India and some of the classical upwelling regions of the world, particularly with reference to two important features, namely, formation and disappearance of temporary banks of mud and the occurrence of special planktonic blooms often leading to mass mortality of fish.