The penetrance in heterozygotes of vestigial mutants was analysed. Two ,genetic modifier systems, one enhancing, the other diminishing the penetrance of vg in heterozygotes were postdated. Both modifiers were found either in the wild or in the mutant strains, and are located on the X chromosome.From the cross of two wild type stocks, a strain with a high percentage of scalloped wings was selected. The vg + alleles and a modifier are probably implicated in the mutant phenotype.