A recent study showed that pneumococcal pherotypes often coexist in the nasopharynx and suggested that the impact of pherotype-mediated fratricide on competition is limited. We determined the impact of pherotype (or competence-stimulating peptide [CSP]) on pneumococcal nasopharyngeal co-colonization. Of 184 nasopharyngeal samples yielding two serotypes, 39.9% contained CSP1 only, 12.6% CSP2 only, and 47.5% had one strain of each pherotype. The observed proportions of concordant and discordant pherotypes (52.5% and 47.5%, respectively), were compared with the ones estimated (53.8% and 46.2%, respectively), and there were no significant differences (p = 0.9, chi(2) test). Our results support the hypothesis that there is a limited role of pherotype in co-colonization.