Bacterial luciferase, derived from a fusion of the lux A and luxB genes of Vibrio harveyi, has been expressed at very high levels in caterpillars and insect cells. The coding sequence for luciferase was inserted into vectors developed in our laboratory which were designed to expedite screening of recombinant virus. These vectors contained the β-galactosidase indicator gene under control of immediate early (IE1), early (ETL), or very late (P10) promoters and a cloning site for inserting the fused luciferase gene next to the polyhedrin promoter. Recombinant baculo-viruses containing the luciferase gene as well as the β-galactosidase gene could be easily selected when Bluogal (β-galactosidase indicator) was included in the plaque assays. Using cells derived from the fall armyworm (Spodopterafrugiperdä), luciferase was strongly expressed very late in infection (48–72 h). The bacterial luciferase assay was sufficiently sensitive that light production could be detected from an extract of a single cell. In addition, live insects, including the cabbage looper (Trichoplusia ni) and saltmarsh caterpillar (Estigmene acred) were infected by mixing recombinant baculovirus into their diet. Cabbage loopers (with an average wet weight of 223 mg) produced at least 195 µg of active luciferase and levels of synthesis peaked between 96–120 h. The results indicate that bacterial luciferase may be used as a reporter of gene expression in