The uvsW gene of bacteriophage T4 is involved in many aspects of phage DNA metabolism, including replication, recombination and repair. To approach the function of uvsW, the structure and expression of the uvsW gene were first explored. Molecular analyses defined the promoter region, the transcriptional start site, and the probable initiation codon. The required promoter region contains a sequence resembling the consensus for T4 late promoters. Furthermore, transcriptional analyses indicated that uvsW is expressed as a late gene, providing a time frame for uvsW action. Several novel observations restrict possible models for uvsW function. A uvsW-deletion mutation reduced overall phage-phage recombination 1.7-fold, but reduced plasmid integration tenfold relative to the wild-type. Thus, the UsvW protein plays a critical role in a specific recombination pathway involving simple reciprocal exchange. One of the most intriguing phenotypes associated with uvsW mutations is the restoration of arrested DNA synthesis caused by mutations that block secondary initiation, the major mode by which replication initiates at late times in wild-type infections. Experiments with plasmid model systems indicate that a uvsW mutation does not restore the arrested DNA synthesis by rescuing secondary initiation directly. Rather, a uvsW mutation appears to allow some alternative mode of late replication, implying that the UvsW protein normally represses this alternative pathway. The rifampicin resistance of uvsW-repressed replication suggests that it involves either tertiary initiation or some novel mode of initiation. Finally, the inappropriate early expression of uvsW from a heterologous promoter blocks most early phage DNA synthesis in a uvsY-mutant infection, suggesting that the UvsW protein is normally the key regulatory factor in the switch from early to late DNA replication. According to this suggestion, the restored late replication in a uvsW mutant is an abnormal continuation of an early mode(s) of replication.