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nucleopolyhedrovirus (SeMNPV) has high potential for development as a bio-insecticide for control of the beet armyworm (S. exiguo). It is highly infectious for S. exiguo larvae and its host range is very narrow. A prerequisite for such application is the possibility of growin 9 this virus in lar9e quantities, e.g. in insect cell lines. It was observed, however, that polyhedra of SeMNPV plaque-purified in Se-UCR1 cells did not cause larval mortality or mor-bidity when fed to S. exiguo larvae. As this suggested a genetic alteration in in vitro produced SeMNPV, comparative restriction analysis of in vitro and in vivo produced SeMNPV DNA was performed. The restriction patterns of viral DNA from several dif-ferent plaques always differed from that of the wild