The surface waters of the northeastern Arabian Sea sustained relatively high chlorophyll a (average 0.81+ or -0.80 mg m sup(-3)) and primary production (average 29.5+ or -23.6 mgC m sup(-3)d sup(-1)) during the early spring intermonsoon 2000. This was caused primarily by a thick algal bloom spread over a vast area between 17-21 degrees N and 66-70 degrees E. Satellite images showed exceptionally high concentration of chlorophyll a in the bloom area, representing the annually occurring 'spring blooms' during February-March. The causative organism of the bloom was the dinoflagellate, Noctiluca scintillans (Dinophyceae: Noctilucidea), symbiotically associated with an autotrophic prasinophyte Pedinomonas noctilucae. The symbiosis between N. scintillans and P. noctilucae is most likely responsible for their explosive growth (average 3 million cells l sup(-1)) over an extensive area, making the northeastern Arabian Sea highly productive (average 607+ or -338 mgC m sup(-2) d sup(-1)) even during an oligotrophic period such as spring intermonsoon.