Recurring mortalities in£icted by the cirolanid isopodCirolana £uviatilis in the juveniles of the Asian seabass, Lates calcarifer, stocked in a cage farm in the Cochin backwaters, from November 2006 to May 2007, were investigated. Hatchery-reared ¢ngerlings (size 8^12 cm) were cultured in £oating cages and mortalities were observed after a rearing period of 1 month. The ¢sh were found dead in the cages, with only the remnants of ¢sh skeleton and all the £esh eaten away. Investigations revealed a very high abundance of isopods, identi¢ed as C. £uviatilis (Crustacea, Malacostraca, Isopoda and Ciroloanidae). During the ¢rst 2 months of stocking, 35% of the ¢ngerlings died, and the cumulative mortality reached 45% in 6 months, considerably reducing the pro¢ts from the venture. Management measures for the control of the isopods are discussed. This is the ¢rst report on the isopod, C. £uviatilis, in£icting serious mortalities in a tropical cage-rearing system in the southwest coast of India.