Two arsenic compounds, sodium arsenite (NaAsO2) and sodium arsenate (Na2HAsO4), were tested for their possible genotoxicity in germinal and somatic cells of Drosophila melanogaster. For germinal cells, the sex-linked recessive lethal test (SLRLT) and the sex chromosome loss test (SCLT) were used. In both tests, a brood scheme of 2-3-3 days was employed. Two routes of administration were used for the SLRLT: adult male injection (0.38, 0.77 mM for sodium arsenite; and 0.54, 1.08 mM for sodium arsenate) and larval feeding (0.008, 0.01, 0.02 mM for sodium arsenite; and 0.01, 0.02 mM for sodium arsenate). For the SCLT the compounds were injected into males. Controls were treated with a solution of 5% sucrose which was employed as solvent. The somatic mutation and recombination test (SMART) was run in the w+/w eye assay as well as in the mwh +/+ flr3 wing test, employing the standard and insecticide-resistant strains. In both tests, third instar larvae were treated for 6 hr with sodium arsenite (0.38, 0.77, 1.15 mM), and sodium arsenate (0.54, 1.34, 2.69 mM). In the SLRLT, both compounds were positive, but they were negative in the SCLT. The genotoxicity of both compounds was localized mainly in somatic cells, in agreement with reports on the carcinogenic potential of arsenical compounds. Sodium arsenite was an order of magnitude more toxic and mutagenic than sodium arsenate. This study confirms the reliability of the Drosophila in vivo system to test the genotoxicity of environmental compounds.