NUMEROUS attempts have been made by several geneticists to induce mutations in Drosophila by treating either eggs, larvae, pupae or adults with various chemicals. The first unquestionably positive results were obtained by C. Auerbach and J. M. Robson, who induced genetic changes in the sperm of males by exposure to the vapours of various mustards. There is very little doubt that the method of treatment developed by Auerbach and Robson contributed significantly to their success. This method can be used with chemical substances that can be handled as vapours.