Early function of a temperature-sensitive mutant of staphylophage 44A HJD was examined during the twenty-five-minute period following infection. Host cell and phage DNA were labeled with C and3H respectively. DNA was separated into linear and covalently closed circular (CCC) forms by density-gradient centrifugation. The host, S. aureus, shows no CCC DNA, and apparently carries no plasmid. Following infection with wild type phage, CCC DNA forms occur in tritiated and 1 C DNA fractions 10 to 15 min after infection. Infection with mutant at permissive temperature also demonstrates CCC DNA with both labels. Infection with mutant at nonpermissive temperature produced no CCC DNA during the first 25 min after infection. The impaired function in this mutant may be a linker protein.