A marine fungus was isolated from the black tiger prawn Penaeus monodon at Nha Trang, Vietnam, on March 20, 2001 and named isolate NJM 0131. The fungus was identified as Haliphthoros milfordensis from the characteristics of asexual reproduction, and its physiological characteristics were investigated. Although the optimum temperature for growth of the isolate was 25°-30°C, the fungus grew at a wide range of temperatures (15°-40°C). H. milfordensis grew well in 50%-100% seawater, but poorly in PYG agar containing 1.0%-5.0% NaCl and KCl. The fungus grew at a wide range of pH (4.0-11.0) with the optimum pH value of 7.0-9.0. The isolate also showed pathogenicity to swimming crab larvae (Portunus trituberculatus) by artificial infection, but mortality was not high. This is the first report of disease in the black tiger prawn P. monodon in Vietnam caused by H. milfordensis.