The mutagenicities of malondialdehyde and formaldehyde were tested by screening each for genetic mosaics of Drosophila melanogaster and by the Muller-5 test for sex-linked recessive lethal mutations. For comparison, the effects of X-rays were also assayed by the above technique. Malondialdehyde, a degradation product of polyunsaturated fatty acids, was found to be a weak mutagen by the above criteria; it induced point mutations and chromosome exchanges at low frequency, as proved by the mosaic test, but failed to induce detectable sex-linked lethality. Formaldehyde was more mutagenic than malondialdehyde; beside induction of mosaic spots it induced sex-linked recessive lethal mutations, but only in the larval testes of Drosophila. Formaldehyde also induced disintegration of the clones. Formaldehyde treatment (feeding larvae with formaldehyde-containing food for about 4 days) was 5 times more mutagenic than malondialdehyde treatment and 5 times less effective than irradiation by 1000 R of X-rays. Wing mosaicism offers a more sensitive way to detect mutagenesis as compared with eye mosaicism. It is suggested that aldehyde-induced mosaic spots derive from mitotic recombination and point mutations.