Mass mortality associated with luminescence has been dbserved in sorne Thai and Ecuadorian penaeid shrimp hatcheries in recent years. Samples collected from hatcheries in these two different areas revealed luminescent sucrose-negative Vibrio strains in larVal rearing tanks where mortality occurred. Pathogenicity trials were carried out to identify the causative agents of mortality and to study the course of infection . . Results showed that strains isolated from Thai hatclleries were more virulent than Ecuadorian strains when tested on different larval stages of Penaeus monodon. The virulence of these strains was also estimated. Trials on other species ofpenaeid shrimp demonstrated the specifie action of Thai strains. Drug sensitivity patterns revealed a high resistance level of these strains to chemotherapeutants used in shrimp hatcheries, such as chlorarnphenicol and oxytetracycline. In order to characterize the virulence factors, in vitro tests were perforrned. Virulence activities of extracellular products (ECPs) and adhesion on cell lines were studied using primary cultures of different tissues of penaeids. No detectable differences were noticed between these straiils using these in vitro tests.