A massive monospecific bloom of toxic marine raphidophyte Chattonella marina was observed off Kochi along the southwest coast of India during late September 2009 with very high cell density (1.59 x 107 cells L-1) and wide spatial distribution. The tear drop shaped cells were 38-65 μm long, 25-30 μm wide and having large number of chloroplasts. Almost 90% of the phytoplankton population was composed of C. marina in the bloom area. Other phytoplankters were few in number represented by Skeletonema costatum, Rhizosolenia spp., Chaetoceros spp., Psuedo-nitzschia spp. etc. and among these Skeletonema costatum was dominant with cell density 2.3 x 104 cells L-1. The concentration of photosynthetic pigment, chlorophyll a was 8.3 μg L-1 in the bloom area. Toxicity test using the alcohol extracts of the Chattonella bloom samples showed characteristic neurotoxic symptoms in fishes leading to mortality