To protect human and ecosystem health, it is necessary to develop sensitive assays and to identify responsive cells and species (and their life stages). In this study, the relative genotoxicity of two inorganic arsenicals: trivalent sodium arsenite (As3+) and pentavalent sodium arsenate (As5+), was evaluated in two cell lines of phylogenetically different origin, using alkaline single-cell gel electrophoresis (i.e., the Comet assay) and the cytokinesis-block micronucleus (MN) assay. The cell lines were the rainbow trout gonad-2 (RTG-2) and Chinese hamster ovary-K1 (CHO-K1) lines. Following optimization and validation of both assays using reference chemicals (i.e., 1–100 μM hydrogen peroxide for the Comet assay and 1–10 mM ethylmethane sulfonate for the MN assay), cells were exposed to 1–10 μM of both arsenicals to determine the relative extent of genetic damage. The unexposed controls showed similar (background) levels of damage in both cell lines and for both assays. Treatment with the arsenicals induced concentration-dependent increases in genetic damage in the two cell lines. Arsenite was more potent than arsenate in inducing DNA strand breaks in the Comet assay; at the highest concentration (10 μM) arsenite produced similar levels of DNA damage in CHO-K1 and RTG-2 cells, while 10 μM arsenate was significantly more genotoxic in RTG-2 cells. MN induction was consistently higher in RTG-2 cells than in CHO-K1 cells, with 10 μM arsenite inducing an approximate 10-fold increase in both cell lines. MN induction also was positively correlated with DNA strand breaks for both arsenicals. Overall, the study demonstrated that the fish cells are more sensitive than the mammalian cells at environmentally realistic concentrations of both arsenicals, with arsenite being more toxic. Environ. Mol. Mutagen. 44:83–89, 2004. © 2004 Wiley-Liss, Inc.