Fifty Kingella kingae organisms, isolated from tonsillar cultures of day care center attendees during an 11-month period, and 60 isolates derived from epidemiologically unrelated individuals, including 19 isolates from respiratory carriers and 41 isolates from patients with invasive infections, were typed by immunoblotting, pulsed-field gel electrophoresis, and ribotyping. One strain, defined by unique immunoblotting, pulsed-field gel electrophoresis, and ribotyping patterns, represented 14 day care isolates (28%) and was frequently isolated during the first half of the follow-up period; a second strain represented 23 (46%) isolates and prevailed during the last 5 months. Children frequently carried the same strain continuously or intermittently for weeks or months, when it was replaced by a new strain. Epidemiologically unrelated organisms showed greater variability, and no strain represented >5% of isolates. The present results support person-to-person transmission of K. kingae among young children in the day care setting.