Taura syndrome (TS) was hypothesized to be caused by a virus and proven experimentally by meeting the criteria of Rivers' postulates. This was accomplished through 3 serial infectivity studies utilizing specific-pathogen-free Penaeus vannamei as the host for the TS virus (TSV). Test animals were infected via intramuscular injection with either a crude or cell-free suspension of the virus. The source of the crude homogenate was TSV-infected Ecuadorian P. vannamei, which were collected during August 1993. Both types of viral inocula caused cumulative mortalities of 73 to 87% among treatment groups. Diagnosis of TS was based on histological analysis of moribund shrimp collected during each experiment. All moribund shrimp, collected between 1 to 3 d post-injection, demonstrated moderate to severe pathognomonic TS lesions. Both gross external and histological lesions, characteristic of chronic phase TS, were observed in 25 to 100% of all survivors. Virions with a buoyant density of approximately 1.337 g/ml icosahedral morphology, and a diameter of 31 to 32 nm, characteristics which suggest that TSV is a member of either the Picornaviridae or Nodaviridae, were recovered from the dead shrimp collected during each of the 3 infectivity studies. Comparisons of TSV samples isolated from naturally infected P. vannamei from Hawaii (USA) and Ecuador indicate that the same virus was responsible for the TS epizootics in both of these shrimp growing regions.