The phytoplankton species composition and their occurrence and abundance at the mouth of the Moheskhali Channel with the Bay of Bengal, Bangladesh were studied for one year, from August 1998 to July 1999. During the study the occurrence of a number of toxin producing harmful algal species, namely, Dinophysis caudata, D. homunculus, D. mitra, Gonyaulax polyedra, Protogonyaulax catenella and Gymnodinium coeruleum and a bloom of D. caudata resulting in fish kill were observed for the first time in the area. During the bloom in August and September the maximum cell density of D. caudata reached 1.1x 106 cells/L. Some hydrographic parameters e.g., temperature, salinity, pH, local rainfall, nitrate-nitrogen and phosphate-phosphorus were recorded and their relationship with the occurrance and abundance of toxic algal flagellates were studied. No human sickness related to the observed algal blooms was reported. In some other coastal areas of Bangladesh, beyond the studied area, algal blooms and related fish mortality during the same period also occurred. Possibly, cysts and/or living cells of the toxic flagellates were present in the area but it was not reported before because no survey was undertaken during the previous years. In 1998 a historic massive monsoon rainfall occurred in many parts of India and more than 70% of the land of Bangladesh. These flood waters carrying nutrients of agricultural, industrial and sewage origin were discharged as runoff through many rivers including the Ganga, Padma and Brammaputa to the Bay of Bengal, Bangladesh, and that was supposed to have triggered the toxic algal bloom.

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