The growth inhibitions and parallel morphological changes due to three arsenic compounds (sodium arsenite, arsenate, and dimethylarsinate) are examined and compared systematically for the first time in cultured mammalian cells. We used suspension‐adapted Chinese hamster ovary (CHO‐S) cells and cultured them under low and high folate conditions to assess whether arsenic toxicity is related to 1‐carbon metabolism. The concentration of arsenite required to produce a 50% growth rate inhibition is increased from 4 μM to 22 μM when the culture medium folate level is raised from 0.2 μM (low folate) to 2.0 μM (high folate). Arsenate is less inhibitory than arsenite to growth; yet, the presence of a high folate concentration also results in a fivefold increase in the cellular tolerance for this anion. A 50% growth inhibition is attained with 30 μM arsenate in 0.2 μM folate medium, while 150 μM is needed in 2.0 μM culture medium. Dimethylarsinate (DMA) is much less inhibitory to cell growth than either of the inorganic forms. DMA concentrations of 1.2–1.5 mM are required to inhibit growth 50% and a high folate medium makes CHO‐S cells slightly more susceptible to its growth inhibition than a low folate medium. All three forms of arsenic cause the cells to increase in size, but the characteristics of the enlargements are different. Edema and mitochondrial enlargement are two of the most obvious changes in cells exposed to arsenite. The nucleus has large irregular, electron dense inclusions after exposure to arsenate, but the cytoplasm is not as swollen in response to this inorganic form. Both inorganic forms of arsenic result in a breakdown of membrane components and cause the ribosomes to detach from the endoplasmic reticulum and aggregate in the cytoplasm. DMA exposure does not result in generalized mitochondrial swelling and loss of endoplasmic reticulum, but it markedly inhibits mitosis. CHO‐S cells treated with 4.0 mM DMA (90% growth inhibition) contain multiple nuclei, 22% mitotic figures, and clusters of intermediate filaments in their cytoplasm.