It is ordinarily considered that the vast majority of mutations in Drosophila are deleterious and non-adaptive. Evidence is presented to show that the vermilion mutant of D. hydei increased in actual number and possibly in relative numbers in a population of D. hydei living under normal conditions. This mutant lived long enough in the locality to appear in several situations and to show the ratio between the sexes of a well-established sex-linked recessive breeding in the general population. It appears to be not distinctly superior nor markedly inferior to the wild type on the average.