A super-repressed mutant of purR (purRS), which encodes arepressor protein controlling expression of purine biosynthetic genes in Salmonella typhimurium, grew very slowly on NCE medium with 10 mg / mL Ade and lactose as sole carbon source (cannot form colonies). However, a phenomenon of late-arising mutations was observed when purRS mutants were spread on NCE+lactose plates and subjected to a prolonged non-lethal selection. The reconstruction experiments of revertants showed that the late-arising "lac+" mutants are not slow growing mutants. Statistical analysis indicated that the distribution of late-arising mutants is Poisson distribution, showing that reversion occurred after plating. The result of co-transductional analysis preliminarily showed that late-arising mutation occurred at selected gene purR or 16 bp PUR box, cis element of structural gene purD. The above results suggest that the phenomenon of late-arising mutation observed by our system is a result of adaptive mutations which are different from random mutations. This is the first time to extend target genes at which adaptive mutations could occur from structural genes involved in carbon metabolism and amino acid biosynthesis to trans regulatory gene coding repressor protein. Our results have provided not only a new proof for generality of adaptive mutations but also a new system for study on adaptive mutations.