Haemophilus influenzae colonizes the upper respiratory tract and can spread causing otitis and sinusitis. This work aimed to study the oropharyngeal carriage rate in healthy < 5-year-old children attending day care centers in Oviedo, Spain in two consecutive years ( January to March 2004-2005. The carriage rate was 42% (400/960) and highly variable among centers (range, 12% to 83%). Isolates were mainly identified as nontypeable H. influenzae (NTHi, 99%). Epidemiologically, 127 different genotypes were identified by PFGE with a minimum of two genotypes per center. One hundred fourteen children (12%) were included in both studies and none of them harbored the same strain over a period of time. The isolates only showed resistance to cotrimoxazol and ampicillin, presenting a shift in the level of ampicillin reduced susceptibility, showing a predominance of PBP3 mutations in 2004 and a predominance of b-lactamase production in 2005. This study proved the great genetic variability of NTHi isolates that present similar genotypic patterns in both years with no long-term carriage of the same strain.