Arsenic-contaminated drinking water from various towns of Comarca Lagunera, Coahuila, Mexico, was tested for its ability to induce sister-chromatid exchanges (SCE) in Vicia faba. 3-h treatments were applied and the differential staining technique of Tempelaar et al. (1982) was used. Atomic absorption spectrophotometry showed that the arsenic concentration in drinking water was 0.11-0.695 ppm, well over the maximum limit of 0.05 ppm (EPA, 1984). In all cases the SCE frequencies were significantly different from the controls. Some concentrations (0.2, 0.3, 0.5 and 1.0 ppm) of sodium arsenate (V) and potassium arsenite (III) were also applied to Vicia faba and all produced significant SCE frequencies, except 0.2 ppm of sodium arsenate.