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Not AvailableHeavy mortality in cage farmed cobia in the southeast coast of Tamil Nadu occurred during the summer of 2016 was investigated. About 60 per cent of the fish died following a few days of high temperature and low wind in the coastal region. The fish had an average size of 27.5 cm length and 200 g weight. The affected fish had focal erythematous lesions on the ventral distended abdomen, peritoneal serosanguinous fluid accumulation, splenomegaly and swollen kidney. Bacteriological examination of the peritoneal fluid, kidney and spleen revealed the presence of Gram negative short rods in large numbers. The bacteria grew as green colonies on thiosulphate citrate bile salt sucrose agar. The tissues were also screened for the presence of nervous necrosis nodavirus by PCR and cell culture but did not yield positive result. The bacterial strains isolated from the tissues were identified as Photobacterium damselae sub sp. damselae using API 20 E microbial identification kit. The species was confirmed by multiplex PCR for 16S rRNA and urease gene. Sequence analysis of 16S rRNA showed 99 per cent homogeneity with P. damselae subsp. damselae. One of the isolates was used for the pathogenicity study in sea bass fingerlings. The experimental infection was done by intraperitoneal administration of 50 micro liters of two dilutions of the culture at 4.2 ×10 raised to the power 5 and 4.2×103 cfu per fish. The experiment revealed that the bacteria were highly pathogenic to seabass and could induce 100 per cent mortality in 36 h. The same bacteria were re-isolated from kidney sample of moribund fish and were confirmed as Pdd with API test kits and 16S rRNA sequencing. The isolates were also screened for the presence of virulence genes.Not Availabl