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A highly intense bloom of the nitrogen fixing Cyanobacteria Trichodesmium erythraeum (~7,000 filaments l-1) was observed during April 2013 (for a period of 5 days) in the Andaman Sea, at Lat.10°-13° N and Long. 93°-95° E.  This is the first report of this bloom in the open waters of this region. Atmospheric temperature at the time ranged from 27 to 30.5 °C, sea surface temperature ranged from 29 to 34 °C, and salinity values ranged from 32.5 to 34 psu. However, there was no significant variation in pH (8.1-8.3), and DO concentrations ranged from 4.7 to 5.5 mg L-1 during the study period. Phosphate values ranged from 0.07 µmol L-1 to 0.57 µ mol L -1, silicate values ranged from 1.7 to 2.7 µmol L-1, nitrate levels were very low (0.3-0.57 µmol L-1). At this time, the biomass of Trichodesmium erythraeum was high, indicating the bloom was in a growth phase. An upsurge in water temperature was found to explain the bloom, together with an increase in salinity. The hooking rate of fish ranged from 0 to 0.32 %. However, in the study area with the highest density of the bloom, almost zero fish catch was recorded, clearly indicating the harmful impact of this algal bloom on fish populations and their distribution pattern.