The occurrence of a large number of diverse arsenic species in the environment and in biological systems makes it important to compare their relative toxicity. The toxicity of arsenic species has been examined in various cell lines using different assays, making comparison difficult. We report real-time cell sensing of two human cell lines to examine the cytotoxicity of fourteen arsenic species: arsenite (AsIII), monomethylarsonous acid (MMAIII) originating from the oxide and iodide forms, dimethylarsinous acid (DMAIII), dimethylarsinic glutathione (DMAGIII), phenylarsine oxide (PAOIII), arsenate (AsV), monomethylarsonic acid (MMAV), dimethylarsinic acid (DMAV), monomethyltrithioarsonate (MMTTAV), dimethylmonothioarsinate (DMMTAV), dimethyldithioarsinate (DMDTAV), 3-nitro-4-hydroxyphenylarsonic acid (Roxarsone, Rox), and 4-aminobenzenearsenic acid (p-arsanilic acid, p-ASA). Cellular responses were measured in real time for 72hr in human lung (A549) and bladder (T24) cells. IC50 values for the arsenicals were determined continuously over the exposure time, giving rise to IC50 histograms and unique cell response profiles. Arsenic accumulation and speciation were analyzed using inductively coupled plasma-mass spectrometry (ICP-MS). On the basis of the 24-hr IC50 values, the relative cytotoxicity of the tested arsenicals was in the following decreasing order: PAOIII≫MMAIII≥DMAIII≥DMAGIII≈DMMTAV≥AsIII≫MMTTAV>AsV>DMDTAV>DMAV>MMAV≥Rox≥p-ASA. Stepwise shapes of cell response profiles for DMAIII, DMAGIII, and DMMTAV coincided with the conversion of these arsenicals to the less toxic pentavalent DMAV. Dynamic monitoring of real-time cellular responses to fourteen arsenicals provided useful information for comparison of their relative cytotoxicity.