It has previously been established that the functional gene A product of phi chi X 174 is required for double-stranded DNA replication and that mutants in gene A affect the lysis of the host cell. We report here other alterations of normal phenotype for a subset of gene A mutants suggesting additional functions of gene A. Mutants in the subset failed to terminate cellular DNA synthesis and were unable to efficiently inactivate the colony-forming ability of the host. Two mutants in a second group retained the ability to kill the infected cell, although only one of these mutants efficiently terminated cellular DNA synthesis. Normal termination of cellular DNA synthesis did not occur by the production of random multiple breaks in the DNA, although it may have occurred by the selective production of breaks in newly synthesized DNA. It has previously been shown that two protein products are produced from the gene A region, the smaller of which is a C-terminal fragment of the larger. The separate phenotypes reported here for the two groups of mutants in gene A are consistent with separate functions for the two gene products previously reported.