The first field study of a genetically altered virus in the United States was performed with an isolate of theAutographa californicanuclear polyhedrosis virus (AcMNPV), which lacks a polyhedrin gene. In the first year of the study, three applications of 7.4 × 1011AcMNPV polyhedra containing 48% genetically altered and 52% wild-type virus particles (co-occluded) were made on a O.1-ha circular plot of cabbage plants. The application area was surrounded by a 0.7-ha circular buffer zone. Before each application, the plants in the application area were infested with 4,500 third-ins tarTrichoplusia ni(Hübner) larvae. After each application, 100% of theT. nitest larvae sampled 5 d after infection were infected with AcMNPV and produced progeny polyhedra containing an average of 42 ± 17.6% genetically altered virus particles. At the end of the 1st yr, the progeny polyhedra population in the application area was estimated at 1.6 × 1013polyhedra. In the 2nd yr, the application and buffer sites were replanted with cabbage plants. At four times during the growing season, the plants were seeded withT. nilarvae or eggs. Less than 2% of the test larvae became infected with AcMNPV. Polyhedra were extracted from soil samples collected in the application and buffer areas. Using neonate larval bioassays with the soil extracts, it was estimated that the soil in the application and buffer areas contained an average of 1,652 ± 3,370 and 832 ± 2,539 biologically active polyhedra per gram dry weight, respectively. Seventy-five larvae infected with polyhedra extracted from application area soil samples produced progeny polyhedra containing a mean of 9 ± 19% genetically altered virus particles. In the 3rd yr, the application area soil samples contained an average of 1,671 ± 3,274 biologically active polyhedra per gram dry weight. Eighty-four progeny polyhedral samples contained a mean of 6 ± 14% genetically altered virus particles. The co-occlusion strategy did not alter the environmental persistence of the polyhedra containing both wild-type and polyhedrin-minus virus particles. However, the data show a decline in the percent of polyhedrin-minus particles in the polyhedra and demonstrate that the persistence of a polyhedrin-minus virus in a cycling virus population is limited by the co-occlusion process. The environmentally desirable attributes of using the co-occlusion process for genetically enhanced baculovirus pesticides and possible problems are discussed.