The extent and duration of transmission of influenza A/New Jersey/76 virus at Fort Dix, New Jersey, was examined with use of titers of hemagglutination-inhibiting antibody to influenza A/Mayo Clinic/74 (Hsw1N1) antigen as an index of infection. Five hundred ninety-three soldiers (a 9.3% sample) in basic combat training (BCT) were grouped in weekly cohorts by the date on which BCT was started. Cohorts with 11 of the 12 BCT cases of A/swine influenza began training on January 12, 19, and 26, 1976, respectively; 9%–19% of the trainees in these cohorts had titers of antibody to A/Mayo Clinic antigen of ⩾1:20. In five other cohorts without cases, which began training before January 12 or after January 26, 0–5% of trainees had titers of ⩾1:20. Titers observed in persons from military units not in BCT and in the civilian population of the post were not consistent with widespread transmission of the virus. Transmission of influenza A/New Jersey virus at Fort Dix probably occurred between mid-January and mid-February with little involvement of people ouside of BCT units.