A prevalence study of personnel on a Royal Naval Air Station revealed that 23·0% of 2479 personnel were carrying a meningococcus. Selected groups of personnel were subsequently swabbed monthly for a year. We have shown that it is only by repeated swabbing and the use of optimal methods including enrichment media that one can have a hope of identifying the ‘true’ carriage rate. A presumed virulent strain of Neisseria meningitidis B 15 P1·16 was repeatedly isolated from three personnel who remained well, as did their colleagues both at their work place and socially. The study served to emphasize our lack of knowledge of the virulence factors associated with N. meningitidis.