The baculovirus protein P143 is essential for viral DNA replication in vivo, likely as a DNA helicase. We have demonstrated that another viral protein, LEF-3, first described as a single-stranded DNA binding protein, is required for transporting P143 into the nuclei of insect cells. Both of these proteins, along with several other early viral proteins, are also essential for DNA replication in transient assays. We now describe the identification, nucleotide sequences, and transcription patterns of the Choristoneura fumiferana nucleopolyhedrovirus (CfMNPV) homologues of p143 and lef-3 and demonstrate that CfMNPV LEF-3 is also responsible for P143 localization to the nucleus. We predicted that the interaction between P143 and LEF-3 might be critical for cross-species complementation of DNA replication. Support for this hypothesis was generated by substitution of heterologous P143 and LEF-3 between two different baculovirus species, Autographa californica nucleopolyhedrovirus and CfMNPV, in transient DNA replication assays. The results suggest that the P143-LEF-3 complex is an important baculovirus replication factor.