MALEIC hydrazide is one of the various reagents which induce breaks in the chromosomes of some plants at specific regions recognizable as heterochromatin1,2. It also inhibits growth in several species of plants, at least doubling the time taken to proceed through a complete mitotic cycle. Much of this delay in cell development has been explained by Evans and Scott3 as due to severe depression in the rate of DNA synthesis. They also suggest that the aberrations are produced at the time of duplication, and that maleic hydrazide acts by interfering, in some way, with the normal process of duplication. It was also found by Compton4 that it has a more pronounced effect on cell division in the roots than in the shoots. In view of its pronounced effects in plants, it was thought of interest to test this chemical for mutagenic action in Drosophila.