The effect of a Haemophilus influenzae type b (Hib) polyribosylribitol phosphate-tetanus toxoid conjugate vaccine (Hib/PRP-T) on oropharyngeal carriage of Hib was studied during an efficacy trial in Gambian infants. Children were vaccinated with Hib/PRP-T and diphtheria-tetanus toxoids-pertussis (DTP) or DTP alone at ages 2, 3, and 4 months. Groups of 1000 children aged 1-2 years were studied each year for 4 years. Hib was detected by production of a halo on antiserum agar plates. Carriage was significantly lower among children fully vaccinated with Hib/PRP-T given with DTP (4.4%; 95% confidence interval [CI], 3.8%-5.7%) than among children fully vaccinated with DTP alone (11.0%; 95% CI, 8.9%-13.0%) (protective effect adjusted by year = 60%; 95% CI, 44%-72%; P < .001). Hib carriage varied by year among nonvaccinated children. Hib conjugate vaccines are likely to produce a herd protective effect in underdeveloped communities, as recorded in Europe and the United States.