Triazole-resistant (Trz(r)) derivatives of six cysteine- or methionine-requiring (Cym(-)) mutants of Salmonella typhimurium were isolated. Some of the derivatives of each mutant (CTS) were prototrophic, i.e., Cym(-) was suppressed. In every case suppression was initially unstable, Cym(-) auxotrophs being segregated at high frequency, although Trz(r) was stable. After several subcultures on selective medium, CTS strains were classified as either persistently unstable or stabilized. The unstable strains segregated Cym( -) auxotrophs at frequencies of 50-70%, whereas the stabilized strains segregated at frequencies of less than 1%. All suppressed strains had a stable Trz(r) marker co-transducible with cysA. However, there was a correlation between the class of CTS strain and Cym(- ) phenotype. The stabilized strains were Cym(+), whereas the unstable strains were Cym(-). Acriflavin and ethidium bromide increased segregation in the unstable strains, suggesting the involvement of a plasmid. The stabilized strains were refractory to the curing agents. There was no detectable change in the quantity or quality of the S. typhimurium cryptic plasmid. The Trz(r) phenotype of the CTS strains suggested that Trz(r) mutations were of the stable TrzA type. It is suggested that correction of the Cym(-) lesions in CTS strains results from an insertion within the cysCDHIJ gene cluster of a DNA species originating in the cysALKptsHI region of the S. typhimurium chromosome.