Arsenic, a common environmental pollutant, is toxic to many mammalian cells. However, the arsenic-induced toxicity to aquatic animal species is unclear. This study attempted to compare the arsenic-induced cytotoxicity in various fish cells. Two fish cell lines, JF (fin cells of Therapon jarbua) and TO-2 cells (ovary cells of Tilapia), were treated with sodium arsenite in two ways to mimic acute and subacute exposure. The distinguishable alterations of cell morphology and microtubule network were observed in the cells treated by two arsenite exposure protocols. By the colony-forming assay, we demonstrated that the survival of both cell lines, treated with the high concentrations of arsenite (20–160 μM) for 2 h or with the low concentrations (0.125–10 μM) for 24 h, was decreased in a dose-dependent manner. The difference between the susceptibility of JF and TO-2 cells to arsenite was revealed by the factorial ANOVA to compare the survival rates of the arsenite-treated cells; JF cells were more sensitive than TO-2 cells ( P=0.008 and 0.013 for the high-concentration and the low-concentration treatment, respectively). The possible mechanisms to provoke the cytotoxicity of arsenite in two cell lines were also addressed. Antioxidants, N-acetyl-cysteine and dithiothreitol, significantly prevented JF cells, but not TO-2 cells, from the arsenite-induced inhibition of survival. Additionally, apparent apoptosis of JF cells and a mitotic arrest of TO-2 cells in response to the treatment of arsenite were also demonstrated by the DNA-fragmentation analysis and the flow cytometric analysis of cell-cycle progression. The results indicate that sodium arsenite induces apoptosis in JF cells probably by causing oxidative stress and disturbs the cell cycle of TO-2 cells. These two fish cell lines can serve as the potential tools to in detail study the toxicity and the hazards of arsenic compounds to aquatic animals at molecular level in the future.