A series of recombinant viruses was constructed to study the regulation of expression from the promoter (P_cap) for the major capsid-protein-encoding gene (vp39) of the baculovirus, Autographa californica nuclear polyhedrosis virus. Each virus of this series contains the cat reporter gene under the control of P_cap or portions thereof. P_cap regulation of cat was determined at the RNA and enzyme activity levels, and was compared to cat gene expression driven by P_polh, the promoter of the polyhedrin gene (polh). Although optimum expression was achieved with a 464-bp region, which included all three P_cap transcription start points (tsp), a 99-bp segment of P_cap containing a single tsp was sufficient to direct late cat expression. P_polh and P_cap-mediated regulation differed temporally; P_cap-cat constructs were expressed at 12 h post-infection (p.i.) and continued through 48 h p.i., whereas P_polh-cat were not expressed at 12 h, but were initiated around 18 h p.i. and underwent a burst of expression between 24 and 48 h p.i. A recombinant virus carrying a hybrid vp39 and polh promoter (P_cappolh) was also constructed and studied; P_cappolh contained the two distal tsp of P_cap and a proximal tsp of P_polhP_cappolh exhibited both ‘late’ and ‘very late’ regulation; the distal tsp were regulated as late sites and the proximal tsp was regulated primarily as a very late site although it also showed a weak late response. Hybrid late/very late promoters should prove useful in optimizing foreign gene expression using baculovirus vectors.