Over a 12-months period respiratory Pseudomonas aeruginosa isolated from CF patients were typed by serology and pyocin production to determine whether cross-infection was occurring. Results of typing were interpreted in relation to the degree of contact patients had with each other. One strain appeared in 4 unrelated patients. However, since none of these patients had been in contact with each other the strains considered to have been acquired from the environment. Each of six pairs of siblings shared the same strain, but the pairs of strains were distinct from each other. These results suggest that the environment is the most important source of Pseudomonas strains for CF patients and that for cross-infection to occur prolonged intimate contact is required.