The distribution of serological types of D. pneumoniae was investigated in 40 strains isolated from 26 children with cystic fibrosis and 57 strains isolated from 39 children with other diseases. All strains were isolated from sputum or tracheal secretion. The strains from cystic fibrosis patients belonged to 14 different serological types, the most prevalent were 19F, 19A and 3. The strains from the other group of children belonged to 20 different serological types, the most prevalent were 23F, 19F and 11A. The differences between the two groups of patients as to the prevalences of types were small, and it is concluded that no special serological types of D. pneumoniae are associated with cystic fibrosis.