This study presents in situ evidence for the blooms of Trichodesmium erythraeum observed in the shelf waters of the South Eastern Arabian Sea (SEAS) during the onset of the southwest monsoon in June 2009. Evidence showed that water surface discoloration was caused by the accumulation of T. erythraeum, and that the water column contained a colony of T. thiebautii. The surface water color in the bloom region varied from pale brown to pinkish red. Pale brown indicated healthy algae at the peak of its photosynthetic activity, while pinkish red indicated the presence of photosynthetically less active filaments. Zooplankton abundance, especially copepodites, in the bloom area substantiated the theory that Trichodesmium filaments are excellent epiphytes to which the copepodites cling. The bloom area was very fertile with copious quantities of dissolved oxygen (6.85 ml L-1), PO4-P (0.108 μmol L-1) and SiO4 (1.29 μmol L-1). Lower NO3-N (0.028 μmol L-1) values in the bloom area did not appear to affect Trichodesmium growth from molecular nitrogen fixation. However, lower NO3-N values altered the normal phytoplankton composition of this area.