We have determined the numbers and types of mutations that occur in strains of Escherichia coli defective in mutT and (or) mutY repair. High rates of C∙G to A∙T mutations in mutY− cells are unaffected by the status of mutT. However, mutT−/mutY− strains have higher rates of A∙T to C∙G mutations than mutT−/mutY− strains. This result indicates that the high rates of A∙T to C∙G mutations seen in mutT− strains of E. coli are due in part to the activity of the mutY repair system. We conclude that mutY repair is mutagenic in a mutT− background.Key words: mutT, mutY, E. coli mutator strains.