A variety of phenotypes of P. aeruginosa can be isolated from the sputum of cystic fibrosis (CF) patients colonised by this organism. Previous studies have described an association between some of these phenotypes and suggested that their emergence was due to conversion or selection by temperate phage. As a result I carried out, (a) a comparative survey of phenotypes in 200 CF and 100 non-CF cultures of P. aeruginosa and a statistical analysis of their association, (b) a study of the rates of mutation for the predominant phenotypes and their genetic linkage, (c) a study of the incidence and properties of temperate phages in CF and non-CF cultures and their association with strain phenotype. A wide range of phenotypes were found in CF and the most frequent were mucoid colony type (50%), atypical serotype (86%), serum sensitivity (74%) and extreme sensitivity to antibiotics and antimicrobial agents, especially to carbenicillin (20%), Irgasan (25%), nalidixic acid (20%), EDTA (20%), cetrimide (10%) and trimethoprim (20%). In contrast, less than 1% of non-CF isolates exhibited these characteristics. Statistical analysis showed that with the exception of antibiotic hypersensitivity, the phenotypes of CF cultures were generally unrelated. Mutation rates determined for the predominant CF phenotypes showed that they were strain dependent and varied between each distinct phenotype. Although some evidence was found for linkage between some of the phenotypes, it was also strain related and the majority of traits varied independently of each other. The incidence of temperate phage was similar in CF and non-CF cultures. Furthermore, the lysogeny state did not influence strain phenotype. P. aeruginosa temperate phages isolated from both collections were similar in host range and serological properties, but were distinct from the typing phages. The selection and examination of the features of phage resistant variants by the use of virulent and temperate phages showed that some cultures exhibited characteristics similar to those isolated from CF. No evidence was found for the role of phage in promoting phenotype change in CF.