During an 18-month period, monthly pharyngeal cultures for Haemophilus influenzae type b (Hib) were obtained from 66 children and their caretakers in a day care center in which no systemic disease caused by Hib occurred. The average colonization rate for Hib was 10%, and ranged from 0% to 23% for a single month. Infants housed in a separate building with a cohorted staff were not colonized by Hib. However, 71% of the toddler group and 48% of the preschool group became colonized by Hib at some time during the 18-month-study. Of 89 Hib isolates, 93% were biotype 1 (Kilian), and 90% of these had a similar outer membrane protein profile, designated subtype 1L. This strain was recovered from children at the center for 15 of 18 months. No invasive disease occurred. Thus, Hib may be widespread among preschool children in a day care center and persist for longer than a year without resulting in systemic disease.