Abstract Sex-linked recessive lethal mutations were induced in Drosophila melanogaster males by gaseous 1,2-dibromoethane at concentrations ranging from 0.2 to 2 parts per million. Significant numbers of mutations could be induced at all these concentrations. Pronounced germ-cell sensitivity differences were observed. For low exposures, spermatids and spermatocytes were about 10–20 times more sensitive than spermatozoa. The dose—effect relation was linear below 60 ppm · h for the 3 cell types. At higher exposures, sterility prevented mutation detection in spermatocytes and in spermatogonia. The lowest effective exposure for spermatozoa was 18 ppm · h (0.25 ppm for 72 h). In spermatids, the lowest exposure tested, 2.3 ppm · h (0.2 ppm for 11 h) induced 4 times the spontaneous mutation rate. Therefore, using prolonged exposure periods one may be able to detect concentrations in the range of parts per billion. Thus, Drosophila appears suitable as a system for detecting very low concentrations of gaseous mutagens in industrial, agricultural and environmental atmospheres.