The vermilion gene was used as a target to determine the mutational specificity of ethyl methanesulfonate (EMS) in germ cells of Drosophila melanogaster. To study the impact of DNA repair on the type of mutations induced, both excision-repair-proficient (exr+) and excision-repair-deficient (exr-) strains were used for the isolation of mutant flies. In all, 28 mutants from the exr+ strain and 24 from the exr- strain, were characterized by sequence analysis. In two mutants obtained from the exr+ strain, small deletions were observed. All other mutations were caused by single base-pair changes. In two mutants double base-pair substitutions had occurred. Of the mutations induced in the exr+ strain, 22 (76%) were GC----AT transitions, 3 (10%) AT----TA transversions, 2 (6%) GC----TA transversions and 2 (6%) were deletions. As in other systems, the mutation spectrum of EMS in Drosophila is dominated by GC----AT transitions. Of the mutations in an exr- background, 12 (48%) were GC----TA transitions, 7 (28%) AT----TA transversions, 5 (20%) GC----TA transversions and 1 (4%) was a AT----GC transition. The significant increase in the contribution of transversion mutations obtained in the absence of an active maternal excision-repair mechanism, clearly indicates efficient repair of N-alkyl adducts (7-ethyl guanine and 3-ethyl adenine) by the excision-repair system in Drosophila germ cells.