The giant freshwater prawn, Macrobrachium rosenbergii (de Man), has been one of the major aquacultural products of Taiwan for the past decade. The annual production of cultured freshwater prawns increased from 1477 MT in 1982 to 16 196 MT in 1991, and in recent years, these animals have become the second most important cultured crustacean crop after the marine prawn, Penaeus monodon (Fabricius). Since 1992, the postlarvae of freshwater prawns in southern Taiwan have been affected by an epizootic disease similar to idiopathic muscle necrosis (IMN) syndrome (Akiyama, Brock & Haley 1982; Anderson, Nash & Shariff 1990b). The affected prawns exhibit white opaque areas in abdominal segments, commonly accompanied by progressive weakening of their feeding and swimming ability. The histopathological changes are similar to those of IMN syndrome, as described by previous studies (Nash, Chinabut & Limsuwan 1987). Unlike IMN, a cytoplasmic inclusion body has also been detected in the necrotic muscle of diseased prawns. Electron microscopy revealed icosahedral virus particles in the cytoplasm of necrotic cells as well as aggregations of viral particles in the inclusion body. The virus is temporarily named Macrobrachium muscle virus (MMV) until its taxonomical position is ascertained by analysing the structure of the genomic DNA. The present paper discusses the first histopathological and ultrastructural observation of MMV infection in cultured freshwater prawn postlarvae from Taiwan.