The locus responsible for mucoid colony morphology in five independent clinical isolates of Pseudomonas aeruginosa from cystic fibrosis patients have been transferred by means of pM060-mediated conjugation to the genetically characterised strain P. aeruginosa PAO. Genetic mapping has shown that in all five strains the locus is on the chromosome between 89' and 94', although it is not possible to say that the same locus is involved in each case. The way is now open for a more detailed genetic analysis of the loci responsible for mucoid colony morphology.