Although its mutagenicity has not been confirmed in mouse germ cells, urethane (ethyl carbamate) gas induces a significant increase of X‐linked recessive lethal mutations in the germ cells of Drosophila melanogaster. The mutation frequency increased as the exposure time was changed from 3.5 to 5.5 h. Mutations were also induced by X‐rays (20 to 40 Gy) and N‐methyl‐N‐nitrosourea (MNU) (0.06 to 0.10%). However, no significant increase of chromosomal changes (partial loss of the Y chromosome, total loss of X or Y, and translations) was produced by urethane, although these were readily induced by X‐rays. There were large and significant increases in chromosomal changes caused by X‐rays (20 Gy) compared to urethane (5.5 h) or MNU (0.06%). In contrast, there were no substantial differences among these three treatments as regards recessive lethal mutations.” Urethane‐induced DNA lesions detected as recessive lethals appear to be intragenic mutations. Complementation analysis with 15 reference single‐site loci (cistrons) in the zeste‐white region of the X chromosome revealed that 29 of 723 urethane‐induced recessive lethals were located in the zeste‐white region and all were restricted to a single locus. However, among 28 of 890 X‐ray‐induced lethals, 2 were non‐complementary to 2 or 3 adjacent loci, indicating deletions encompassing 2 or 3 loci. In addition, 3 of these lethal chromosomes included mutations outside the zeste‐white region. Another difference between urethane and X‐rays was in the distribution of mutation sites. Urethane‐induced mutations were strikingly non‐random with two hot spots at zw‐1 and zw‐2, whereas the distribution of X‐ray‐induced mutations was more nearly random.