Esterase electrophoretic typing and restriction fragment length polymorphism of ribosomal DNA regions (ribotyping) were used to differentiate 102 Pseudomonas aeruginosa clinical isolates obtained from chronic lung infection in 23 patients with cystic fibrosis (CF) and two reference strains (including the type strain ATCC 10145). Twenty-five zymotypes were obtained with the former method and 16 ribotypes with the latter. Combination of the two typing systems led to the finding of 30 different types. Our data highlights the physiopathological complexity of P. aeruginosa infection in CF as, in six individual cases, several types were found among isolates from a given patient. On the other hand, two unique types were found in two and three patients respectively, raising the possibility of crossinfections.