Promoter function of the putative polyhedrin-encoding gene (polh) of Spodoptera litura nuclear polyhedrosis virus (SIMNPV) was determined by transferring it to the Autographia californica nuclear polyhedrosis virus (AcMNPV) through the AcNPV polh based vector, pVL1393. Three transfer vectors pCBT2, pCBT3 and pCBT4 were constructed by substituting the promoter and the neighbouring sequences of AcNPV in pVL1393 by that of SINPV. The Escherichia coli lacZ gene was placed downstream from the SINPV polh promoter in the hybrid transfer vector (pCBT) constructs. Co-transfection of Spodoptera frugiperda cells (Sf9) with each of the pCBTlacZ vector and wild-type AcNPV DNAs led to synthesis of β-galactosidase (βGal). The plaque-purified recombinant viruses (S1AcNPV.lacZ) expressing lacZ under the polh promoter of SINPV are stable. The highest βGal activity was obtained with S1AcNPV4.lacZ. Production of βGal with recombinant virus, S1AcNPV3.lacZ in which SINPV polh promoter is in the reverse orientation in the AcNPV genome, is 83% of that produced by S1AcNPV4.lacZ. These results indicate that the SINPV polh promoter is active in the genetic environment of AcNPV; the polh of SINPV is phylogenetically related to AcNPV like other baculoviruses.