An explosive outbreak of influenza caused by an A/USSR/77-like (H1N1) virus occurred aboard a US Navy ship in December 1977 and January 1978. Two hundred volunteers aboard the ship were studied. Virus was isolated from 36 of 57 patients from whom isolation was attempted. Among virologically confirmed patients, headache (97%), chills (92%), malaise (86%), and cough (75%) were the most frequent symptoms. There were two virologically confirmed cases with complications: one with collapsed lung and the other with pneumonitis. The study subjects were 25 years of age or less, but there was little influenza-like disease in members of the crew greater than 25 years of age. Prior vaccination with bivalent vaccine, containing A/NJ/76 (Hsw1N1) virus, did not offer significant protection against disease caused by A/USSR/77-like virus. Serologic tests, either or both complement fixation and hemagglutination inhibition, were positive in only 14 of 22 virologically confirmed cases, indicating a poor serologic response to primary infection with this strain of virus. These findings prevented calculation of meaningful disease to infection ratios. However, inapparent infection occurred in 3 of 19 (16%) individuals who denied having illness during the outbreak yet had serologic evidence of recent influenza infection.