It was controversial issue if nuclear polyhedrosis virus(NPV)could replicate in midgut cells of host larvae from Lepidoptera by now.The replication of Mythimna separata NPV (MsNPV) in M.separata larvae midgut cells was studied by ultrastructural and DNA hybridized techniques.The paper demonstrated that the MsNPV could neither infect midgut cell nor replicate in midgut cell of homologous host.Therefore MsNPV virions released from the virial occlusion bodies were considered as direct penetration though the intercellular spaces of midgut cells to hemocoel of the host larvae.