Abstract
Evidence is presented that X-rays and gamma rays can induce back mutations which, insofar as current criteria permit, do not differ from spontaneous back mutations. D. melanogaster [female][female] homo-zygous for one or the other forked pseudoalleles, f3n and f1, both known to back-mutate spontaneously, were irradiated with 4000 r X-rays or gamma rays. The reversal frequency of f3n was significantly increased; experiments with f1 showed no comparable increase. No gross chromosomal rearrangements were associated with the reversions.