Fifty-six non-haemolytic Haemophilus strains were isolated during current bacteriological examinations from material from the lower respiratory tract of 39 patients with cystic fibrosis during a 6-month period. Except for six strains which belonged to Haemophilus parainfluenzae, all strains were identified as Haemophilus influenzae. Biotype I of H. influenzae was the predominating taxon (38%) and was especially related to patients with recurrent or prolonged colonization. Only two strains were capsulated. An unexpectedly high percentage of the strains produced lysine decarboxylase. The significance of this property in the pathogenesis of the respiratory tract colonization is discussed.