A total of 5217 middle ear fluid (MEF) samples obtained from 1203 children with otitis media, aged 3 months to 6 years, were studied for the presence of Branhamella catarrhalis (Br) between Oct. 1977 and Sept. 1981. Br grew in 10.2% of 3497 MEFs of acute otitis media (AOM), with almost the same frequency in the very first and subsequent attacks. During the first 4 years of life the percentage did not vary much; among older children it seemed to decrease. The overall prevalence of Br in AOM did not change during the study period. Br alone grew in 72.4% of acute MEFs with Br; with other bacteria the respective figure was 82.9% (p less than 0.001). Acute attacks with bilateral Br were found in 22.8% of attacks with Br. In 1720 non-acute MEFs obtained at postacute control visits, Br was isolated in only 7.0%. The proportion of beta-lactamase-producing strains among the 2419 otitis-Br strains tested in two laboratories of the two study regions showed an increase from 27.1% and 21.1% in 1980 to 57.6% and 38.6% in 1983, respectively (p less than 0.001).