The bacteriologic findings of 1205 episodes of pneumococcal otitis media are analyzed. Capsular types 6, 14, 19 and 23 have been found to account for more than half the initial and subsequent infections of the middle ear. Initial infection has been shown to be related to age, and recurrent infection with pneumococci of heterologous capsular types has been found to occur most often within six months of the initial attack. Nasopharyngeal carriage of the pneumococcal types most frequently causing otitis media may occur without causing infection, may antedate infection by as long as 13 months and may persist after infection for as long as 30 months. Recurrence of infection with a given capsular type has been identified following intervening infection with one or more heterologous capsular types. Simultaneous infection of the middle ear with two pneumococcal capsular types has been observed in 1% of the infections studied. The potential, but as yet unestablished, value of prophylactic vaccination against pneumococcal otitis media is considered.