The epidemiology of infection due to Neisseria meningitidis and Neisseria lactamica was studied in a northern Nigerian community. A low meningococcal carriage rate was observed throughout the two-year survey. Initially, most meningococci isolated from nasopharyngeal carriers belonged to serogroup C or to serogroup Y. Following an outbreak of group A meningococcal disease, more group A meningococcal carriers were detected. Antibody studies indicated that infection with group A meningococci had been more widespread in the community than was suggested by regular carrier surveys. Carriage of meningococci was detected most frequently in children one to nine years of age. Children were identified as the first carriers in households more frequently than adults. The half-life of carriage was three months. The meningococcal carriage rate did not increase during the hot dry season when epidemics of meningococcal disease occur most frequently in Nigeria. Neisseria lactamica was isolated from the nasopharynx of children more frequently than were meningococci.