Suppressor mutations of Pseudomonas aeruginosa have been identified. An isolate of strain PAT, initially selected as being temperature sensitive for growth, was found to suppress two different auxotrophic mutations. A suppressor locus, designated sup-1, has been mapped and is co-transducible with three closely linked thr loci. The suppressor mutation has been used to isolate suppressor-sensitive (sus) mutants of the virulent phage E79 and the R factor R18. By selecting for revertants of auxotrophic markers, other sup mutants have been isolated and are found to be of two types, either temperature sensitive for growth like the original mutant or showing wild-type growth at 43 C. The mutations giving rise to both these classes of suppressor are very closely linked. One of the sup-1 alleles of strain PAT also shows suppressor activity when transferred into P. aeruginosa strain PAO. Escherichia coli strains carrying the nonsense suppressors supC, supD, or supF do not suppress the sus mutant of R18. This suggests that sup-1 is different from the amber and ochre suppressors of the enterobacteria.