Preliminary paper. On exposure to x-rays, portions of chromosomes become detached and lost or permanently attached to other chromosomes; e.g., part of II may become attached to III. Several frequent translocations of this general character are described. They may be detected by genetical tests, i.e., by new linkage relationships in the progeny of irradiated flies. Cytological studies of flies of these irradiated stocks have, in general, shown actual translocations of parts of chromosomes, corresponding well with the translocations inferred from linkage evidence. This is evidence for the accuracy of the chromosome maps.