This paper describes the genetic analysis of X-ray-induced mutations at several visible loci (yellow, white, Notch, vermilion and forked) located on the X-chromosome of Drosophila melanogaster after recovery in excision repair-deficient condition (mus-201). A total of 118 mutations observed in 83636 F1 females were analyzed. The white mutations in particular have been investigated at the molecular level. The results show that: (1) the frequency of recovered whole-body mutations is similar or slightly lower in repair-deficient than in repair-proficient condition (respectively 1.5 x 10(-4)/locus/15 Gy and 2.3 x 10(-4)/locus/15 Gy); (2) the frequency of observed mosaic mutations is significantly higher in the repair-deficient condition than in the proficient condition (respectively 2.7 x 10(-4)/locus/15 Gy and 0.9 x 10(-4)/locus/15 Gy); (3) the analysis of F2 male lethal mutations and the cytological analysis of the recovered mutations in the excision repair-deficient condition indicate a decrease in mutations associated with gross chromosomal aberrations (including multilocus deletions); (4) at the molecular level, the spectrum of recovered intragenic mutations is similar after excision-deficient and -proficient repair. These results indicate that excision repair is involved in X-ray-induced DNA damage that is repaired efficiently in the normal repair condition, but bypassed in the excision repair-deficient condition, leading to mosaic mutations. In addition, lesions that apparently cannot be bypassed by DNA replication lead to a decrease in the fraction of mutations due to gross chromosomal aberrations among the whole-body mutations.