A seroepidemiologic surveillance of rubella infections in arriving male and female Navy recruits indicated that from 82-90% of young adults have hemagglutination (HA) titers of 1:10 or greater. Subsequent to the establishment of a new male recruit camp, there was a 10- to 11-month hiatus of low level rubella infection in the population. Once endemic rubella was established, more than 73.3 per cent of those with no detectable initial titer seroconverted. There was no reported rubella infection at an isolated female recruit camp of about 500 women even though the proportion of susceptibles was similar. However, once the female recruit camp was relocated within the male camp, their experience with rubella became comparable. A small (6.8% men, 5.6% women) number of recruits with pre-existing HA antibody titers had a four-fold or greater rise in titer during training. The great transmissibility of rubella virus in a closed population was illustrated and the ineffectiveness of herd immunity in this setting was evident.