The quantitative bacteriology of the adenoid was studied in otitis-prone and non-otitis-prone children. alpha-hemolytic Streptococci (Viridans Streptococci) appeared to be predominant normal flora in the healthy nasopharynx. There was a decrease in alpha-hemolytic Streptococci in the otitis-prone child compared to the non-otitis-prone child. Concomitantly, there appears to be an increase in both nontypable Haemophilus influenzae (NTHI) and S. pneumoniae in the nasopharyngeal flora in the otitis-prone child. The mechanisms responsible for this alteration of the micro-ecology of bacteria of the nasopharynx may be related, in part, to factors that alter mucociliary function. These factors could be viral infection, allergy, local and systemic immunological deficiency and the indiscriminate use of antibiotics. An understanding of the relationship between the normal flora and the potential pathogens may be important in the understanding of both the pathogenesis of otitis media (OM) and possibly the treatment of this disease entity.