A monospecific bloom of thecate dinoflagellate Gonyaulax polygramma (5x 10 8 cells L -1 ) was observed from the shelf waters off Mangalore (South Eastern Arabian Sea) during late summer monsoon. The bloom was observed to be subsurface (~5m depth) with chlorophyll a maxima of 13.15 μg L -1 . The species observed to be photosynthetic and no visible food vacuoles were observed within the cell. Water column stability with sufficient inorganic nutrients during the fag end of summer monsoon might have favoured the bloom species. The region observed complete exclusion of zooplankton and can suggestively due to unpalatability of the bloom species.