Chemical treatment of a nuclear polyhedrosis virus ofAutographa californica(Speyer) was carried out initially inTrichoplusia ni(Hübner),Heliothis zea(Boddie),H. virescens(F.), andSpodoptera frugiperda(J. E. Smith), and subsequent effects of treatment on virulence of the virus were assessed in each host. No significant effects were noted in any of the four insect species after treatment with 5-bromodeoxyuridine or nitrosoguanidine. Almost a 100-fold increase in virulence was observed, however, when the virus was treated inT. niwith 3-methylcholanthrene and assayed inS. frugiperda. This isolate also maintained a high degree of pathogenicity forT. ni. Significant changes were also observed in the LT50of several virus samples, but none of these changes persisted after several serial passages. These observations support use of such low-passage virus isolates, which demonstrate a significant increase in virulence, on crops affected with a particular insect pest.