Thioarsenic species are often the predominant arsenic species in sulfidic environments, yet little is known about their toxicity. We report the first determination of acute toxicity of mono-, di-, and trithioarsenate to the bioluminescent bacterium Vibrio fischeri, which increases with an increasing number of thio(SH)-groups. While mono- and dithioarsenate are much less toxic (effective analyte concentration causing a 50 % luminescence decrease (EC50) 676 and 158 mg/L), the toxicity of trithioarsenate (EC50 14.4 mg/L) is comparable to those of arsenate and arsenite (EC50 9.1 and 26.1 mg/L). The low toxicity of monothioarsenate is remarkable, because it has very similar chemical properties to arsenate. In contrast to arsenite and arsenate, the toxicity of thioarsenates increases with exposure time, suggesting a lack of detoxification mechanisms or conversion of thioarsenic species into arsenic oxyanions after uptake. We determined the acute toxicity of synthetic arsenite solutions with varying sulfide concentration to Vibrio fischeri. Arsenic speciation in these solutions was measured by ion chromatography-inductively-coupled plasma-mass spectrometry, and the observed toxicity was related to the different arsenic species present. High luminescence inhibition was observed at low and high sulfur/arsenic ratios, where arsenite or a mixture of di-, tri-, and tetrathioarsenate, respectively, dominated arsenic speciation. Acute toxicity decreased at sulfur/arsenic ratios of 1 to 10, with a minimum of 30 % luminescence inhibition at 3.5, where 55 mg/L arsenite and 30 mg/L trithioarsenate were determined. The toxicity observed under these conditions is much lower than anticipated from the individual dose-response curves which predict that each species alone should already cause 70 to 80 % inhibition. The low toxicity suggests an antagonistic toxicological interaction between arsenite and trithioarsenite.