The biological variability of the northwestern Arabian Sea during the 1979 southwest monsoon has been investigated by the synthesis of satellite ocean color remote sensing with analysis of in situ hydrographic and meterological data sets and the results of wind-driven modeling of upper ocean circulation. The phytoplankton bloom in the northwestern Arabian Sea peaked during August-September, extended from the Oman coast to about 65¿E, and lagged the development of open-sea upwelling by at least 1 month. In total, the pigment distributions, hydrogrphaic data, and model results all suggest that the bloom was driven by spatially distinct upward nutrient fluxes to the euphotic zone forced by the physical processes of coastal upwelling and offshore Ekman pumping. Coastal upwelling was evident from May through September, yielded the most extreme concentrations of phytoplankton biomass, and along the Arabian coast was limited to the continental shelf in the promotion of high concentrations of phytoplankton. Upward Ekman pumping to the northwest of the Somali Jet axis stimulated the development of a broad open-sea phytoplankton bloom oceanward of the Oman shelf. Vertical mxing during the 1979 southwest monsoon was apparently not a primary cause of the regional-scale phytoplankton bloom. ¿American Geophysical Union 1991