Severe epizootic causing high mortalities in cultivated Penaeus chinensis has occurred every year in mainland China since 1993 with great economic loss. Naturally diseased P. chinensis sampled in Qingdao shrimp farming regions between 1993 and 1998 were examined by electron microscopy. A similar non-occluded nuclear bacilliform virus like the white spot baculo-like virus (WSBV) reported in Taiwan was mostly found in the gill, cuticular epidermis under the exoskeleton, stomach, lymphoid organ and other tissues of ectodermal and mesodermal origin from these diseased samples. The viron was enveloped with a mean size of (125 +/- 7.6) nm x (345 +/- 16) nm. Sometimes, a nipple-like appendage could be found protruding from one extremity of the viron. Envelopes were all lost after purification from cesium chloride gradient ultracentrifugation. The cylindrical nucleocapsid in negatively staining preparations was (80 +/- 13) nm x (380 +/- 24) nm with 13-16 stripes perpendicular to the longitudinal axis of the nucleocapsid. Based on its pathogenic and morphological characteristics, this virus should be related to WSBV. A pair of primers created from information of WSBV genome DNA SalI fragment produced a 355 bp band using the WSBV isolate from P. chinensis in mainland China, as the DNA template. The specific PCR product was cloned, sequenced and labeled with digoxigenin (DIG) DNA labeling kit (Boehringer Mannherm). The sequence of the probe is identical to that of WSBV. All of the sampled diseased shrimps between 1993 and 1998 reacted with the DIG-labeled probe by dot-blot hybridization and no hybridization was observed using DNA from health shrimp as templates. These results based on electron microscopy and DNA hybridization indicated that the shrimp epizootic occurred in mainland China was related to the WSBV infection previously occurred in Taiwan early 1992.