Marine organisms frequently contain arsenic in higher concentrations than terrestrial organisms. The water-soluble arsenoriboses are the major organic arsenic compounds in marine algae. We synthesized an arsenosugar, 1-(2′, 3′-dihydroxypropyl)-5-deoxyribosyldime-thylarsine oxide, and examined this compound for cytotoxicity (50%-inhibitory dose; ID₅₀) and for the ability to induce chromosomal aberration and sister chromatid exchanges (SCE). The arsenosugar inhibited the growth of BALB/c 3T3 cells at a concentration of 2mg arsenosugar/ml in the cytotoxicity test. The cytotoxicity of this compound was 1/2800th of that of sodium arsenite and 1/300th of that of sodium arsenate. The aberrations caused by the arsenosugar at a concentration of 5mg/ml consisted mainly of chromatid gaps and chromatid breaks. No SCE was observed at concentrations of 1 to 10mg/ml. These results suggest that the arsenosugar present in marine algae is much less toxic than inorganic arsenic compounds.