The authors report six cases of acute urethritis and three cases of acute cervicitis caused in all instances by Neisseria meningitidis group B. The patients, five heterosexual men, one homosexual man, and three female prostitutes were seen at a venereal clinic in Mexico City. All of them were initially diagnosed as having gonorrhea. Treatment with procaine-penicillin G cured all cases, both clinically and bacteriologically. These experiences reinforce the need to distinguish N. meningitidis from N. gonorrhoeae in the setting of sexually transmitted diseases.