A nucleopolyhedrovirus isolated from the beet armyworm Spodoptera exigua (Polish laboratory culture), SeMNPV (P), morphologically similar to the viral bioinsecticide virus Spod-XR, was characterized molecularly and biologically. Phylogenetic analysis based on three conserved baculovirus genes, polh, lef-8 and pif-2, showed the highest homology of SeMNPV (P) to Mamestra brassicae (Mb) MNPV and M. configurata (Maco) MNPV, and much less to SeMNPV (Spod-XR). These findings were confirmed by genomic DNA restriction profile analyses. Bioassays revealed that SeMNPV isolated from the commercial bioinsecticide Spod-X^R was themost infectious for S. exigua, while the infectivity of SeMNPV (P) and MbMNPV was significantly lower. These data suggest that SeMNPV (P) is a variant of MbMNPV.