A total of eight isolates, including variants and natural recombinants ofAutographa californicaMNPV (AcMNPY) for which genetic markers or physical maps are known were tested for in vivo virulence, and maintenance of genetic characteristics after infection ofTrichoplusia ni(Hübner) (Lepidoptera: Noctuidae) andHeliothis virescens(F.) (Lepidoptera: Noctuidae) larvae. Only one of the isolates was more virulent toT. nithan the wild type; however it was less virulent toH. virescens. One recombinant (AR-66) exhibited low production of polyhedra in TN368 cells, which were used for production of the polyhedra. One variant (AcMNPY-S1) with an insertion of DNA when compared with the AcMNPVE2 variant was at least 9.2 times less virulent than the wild isolate. After one in vivo passage of AcMNPY-S1, a new variant (AcMNPY-S1a) was detected with a new Eco-R1 fragment of 2.17 Mdal and a corresponding loss of a 1.55-Mdal Eco-R1 fragment found in the parental type.