Mass mortality of stinging catfish, Heteropneustes fossilis, was observed in a fish farm of Kulgaria, Burdwan in West Bengal, India. The cumulative mortality rates reached up to 5% of the total fish in the farm per day. The clinical signs of the affected fish showed ulcers on the skin which gradually grew in size and progressed to form ulcerous dermatitis. Clinical signs of catfish experimentally infected with the isolate were similar to those observed in the affected fish of the fish farm. The isolate was identified as Bacillus cereus through biochemical tests and analysis of the isolate by 16S rDNA sequences (97%). With dip treatment (10 min/day) from day 5 to day 15 with Argemone mexicana 1 g/L chloroform: methanol seed extracts, the mortality rate decreased and the treated fish gradually recovered on day 15 and their ulcers completely healed on day 21. The hematological values attained following treatment were very close to that of the normal values (p>0.05). This study is the first time report of B. cereus acting as a pathogen in fish and causing mortality in any fish farm.