White spot baculovirus (WSBV) is the causative agent of a disease which decimated some cultured penaeid shrimp populations and inflicted severe economic damage in Taiwan. Until very recently, the giant freshwater prawn Macrobrachium rosenbergii was thought to be unaffected by this virus, but now signs closely resembling white spot syndrome (WSS) have been observed on its exoskeleton. In this paper, WSBV was established as the causative agent by using the diagnostic polymerase chain reaction (PCR) with WSBV-specific primers. WSBV was found in M. rosenbergii larvae, postlarvae, juveniles, and adults. The amplified product from the DNA of the naturally-infected WSS M. rosenbergii was similar to that of WSBV-infected Penaeus monodon. Furthermore, comparison of the restriction profiles of these two PCR products by HaeIII, HpaII, RsaI, and Sau3AI revealed no differences, suggesting that WSBVs from the infected P. monodon and M. rosenbergii are closely related, if not identical. A homogenate positive in one-step WSBV diagnostic PCR was prepared from frozen P. monodon for the challenge experiment. Dilutions were added to tanks of healthy M. rosenbergii larvae and postlarvae. After 2 days, some of the dead specimens were positive for WSBV by diagnostic PCR.