Mustard gas, ClarkI(diphenylarsine chloride) and Clark II (diphenylarsine cyanide) were investigated for their acute toxic effects on Daphnia magna in brackish water. Exposure to 0.5 mg mustard gas/l test medium for 48 h at room temperature (19,5oC) did not induce any visible effects. The Clark compounds were tested for acute toxicity at three different temperatures: 4.0, 8.0 and 19,5oC. After 48 h exposure the EC50-values were significantly higher in the low temperature experiments than in the high temperature experiment. However, after 6 days the EC50- values were nearly the same for all temperatures. This suggests that the minimum EC50-value of a substance is independent of temperature and that the important factor is the exposure time. Additionally, a sediment experiment conducted with Clark I indicated that Clark I adsorbs to the sediment. After the Second World War approximately 40 000 tons of chemical munitions, mainly containing mustard gas, were dumped in the Baltic Sea. With the results from the experiments and available literature, a risk assessment of the dumped chemical warfare agents was conducted. The conclusions from this risk assessment were that mustard gas does not appear to substances constitute a potential risk to these organisms.