Abstract: A new epidemic spread out recently years in the larvae and post larvae of giant freshwater prawn, Macrobrachium rosenbergii, in many hatcheries of some South China provinces.The diseases,with the symptom of whitish muscle in abdomen or opacity muscles,was called as whitish muscle diseases,causing serious loss of M. rosenbergii larvae or post larvae. The typical diseased post larvae were used for bacteria isolation, tissue homogenizing and artificial challenging test with health M. rosenbergii larvae by immersion infection tests.The health larvae could not be infected by the bacteria strains isolated from hepatopacreas and muscles of diseased larvae in two weeks after infection at the concentration of 5?0 8 CFU/ml,but could be infected by bacteria free tissue homogenate at concentration of 1:50 and 1:250,with symptom similar of the nature diseased shrimps,7 days after challenging.The infectivity could not be inactived by 10% chloroform. With the ultrathin section, spherical virions were found with the size of 21-23nm in the muscle connection. The virus partly purification was carried out with diseased larvae homogenized in PBS,treated with chloroform followed by PEG precipitation and 2% PTA negative staining.It could be found plenty of spherical virions with the size of 22-24nm.These results showed that the whitish muscle disease of M. rosenbergii post larvae was caused by this virus.