Three isolates of gypsy moth [Lymantria dispar (L.)] nucleopolyhedrovirus (LdMNPV), from China (LdMNPV-H), Japan (LdMNPV-J) and the registered strain from North America (LdMNPV-D), were bioassayed in the Inner Mongolia Autonomous Region, People's Republic of China, in 2004, using a modified version of the diet plug method, to determine their pathogenicity and virulence based on the dose-and time-response of second-instar larvae of the Chinese strain of the Asian gypsy moth. Results showed that LdMNPV-H and LdMNPV-D did not differ significantly in dose-response. The LD 50 and LD 95 for LdMNPV-H were 211 and 1414 OBs larva -1 , respectively, while LdMNPV-D had a somewhat lower LD 50 (194 OBs larva -1 ) and a higher LD 95 (1705 OBs larva -1 ). LdMNPV-J was the least pathogenic, with a LD 50 of 940 OBs larva -1 and LD 95 of 11 457 OBs larva -1 . Overall, LdMNPV-H and LdMNPV-D had similar pathogenicity against second-instar larvae of the Chinese strain of the Asian gypsy moth, and were considerably more pathogenic than LdMNPV-J. Time-responses were not significantly different among the three virus strains.