During mortality outbreaks in hatchery-reared Macrobrachium rosenbergii postlarvae (PL) in Guadeloupe Island (French West Indies) during 1997, an associated viral disease was discovered and the agent was subsequently isolated. The clinical signs presented by severely affected PL consisted essentially of an opaque whitish appearance of the abdomen. Histopathological changes in affected PL were characterized predominantly by pale to darkly basophilic, often reticulated, cytoplasmic inclusions in the connective tissue cells of most organs and tissues. The isolated virus was approximately 30 nm in diameter as observed with an electron microscope by negative staining. By its location, structure and size it could be related to different families of the small RNA cytoplasmic viruses such as the Picornaviridae or the Nodaviridae. Its characterization is in progress.