A phytoplankton bloom dominated by Chaetoceros spp. was detected in the Arabian Sea, off Cochin coast during May 2009 coinciding the onset of southwest monsoon with an average chlorophyll a concentration of 8.42 mg m -3 at four stations (A, C, D & E) of total 7 stations studied in coastal waters situated between latitudes 9º54'-10º02' N and longitudes 76º05'-76º12' E. All the stations were resampled after seven days and found that the average chlorophyll a concentration at the bloom stations has decreased to 2.91 mg m -3 . The inherent optical properties (chlorophyll a, specific phytoplankton absorption coefficient, chlorophyll scattering coefficient) of water were analyzed from the study area. The physico-chemical parameters like dissolved oxygen, pH, turbidity and nutrients were estimated for surface waters in these stations using standard methods. The phytoplankton of the study area was identified and their density was recorded. The phytoplankton community composed primarily of diatom Chaetoceros sp. in all the stations, with a percentage composition greater than 55 at the bloom. The highest phytoplankton density recorded was 4,89,578 cell no./l at station D. Samples from the bloom showed higher specific phytoplankton absorption coefficient [a* c (λ)] than other stations. a* c (435) varied from 4.11 to 5.68 m 2 mg -1 and a* c (665) from 1.45 to 1.80 m 2 mg -1 . A survey on purse seine fishery showed a higher pelagic catch in close proximity to the bloom area during this period. Further investigation of absorption properties of blooms using remote sensing at complex case II waters is needed to interpret bio-optical properties as well as fisheries. The present study is the first attempt of this kind off Cochin coast.