In the course of experimental infection of Penaeus monodon with white spot syndrome virus (WSSV) for virus isolation and purification, one unexpected virus was found to be dominant in purified virus samples prepared from shrimp hemolymph. Under electron microscopical observation, the virus was enveloped, rod-shaped and measured 140-200 nm x 35-50 nm. Pure virions with a density of 1.154-1.162 g/mL in sucrose gradient were obtained and at least three viral structural proteins of 20, 63 and 110 kDa were observed when analyzed by SDS-PAGE. Injection of P. monodon with this purified virus demonstrated rapid and mass mortality. By ordinary histological studies, the cells with densely basophilic inclusions were found in the lymphoid organ, gill, stomach, hepatopancreas and heart. By electron microscopy, ultrathin sections of lymphoid organs from infected shrimp showed that numerous enveloped virions and nucleocapsids scattered or enclosed in the vesicles within the cytoplasm. The morphology, histopathology and structural proteins of this virus closely resemble those of the yellow head virus (YHV) from Thailand. Moreover, the results of RT-PCR diagnosis using primers specific to the 135 bp of YHV also suggest that this virus is closely related to YHV. PCR diagnosis of YHV and WSSV in P. monodon sampled from culture farms in Taiwan between 1996 and 1999 demonstrated that YHV has been present in P. monodon for at least 3 years and that all of the samples, which were diagnosed as YHV positive, were also co-infected with WSSV.