The integrase (INT) family of bacteriophage coded integrase-recombinase proteins are responsible for catalyzing strand exchange between DNA molecules and play an important role in the DNA replication of many bacteriophages. Within the frog virus 3 (FV3) genome we have identified an open reading frame (ORF) of which the deduced amino acid sequence contains a motif characteristic of the INT family of integrases-recombinases. The ORF consists of 825 bp which codes for a protein of 275 amino acids with a predicted Mr of 29,945. RNA transcribed from this ORF during virus infection was detected by Northern blot analysis and it is a delayed early message of approximately 1100 bases. The 5' and 3' ends of the putative FV3 integrase-recombinase transcript were mapped. The transcriptional start site is preceded by a presumptive TATA box, and a region of hyphenated dyad symmetry is present at the 3' end of the message. A protein with an Mr of approximately 30,500 was synthesized by a rabbit reticulocyte lysate programmed with capped runoff transcripts from the cloned gene, indicating that the ORF can be transcribed into a message coding for a viral protein. In the FV3 life cycle, DNA replication occurs in a large complex formed through the recombination of small viral DNA molecules. Thus, at this stage, DNA replication and recombination are interlinked. Resolution of concatameric DNA is required for the packaging of genomes into virus particles. The putative FV3 INT gene may be involved in one or more of these functions.