A nitro musk (musk ketone). and three polycyclic musks (Tonalide(TM), Galaxolide(TM) and Celestolide(TM)) were tested for acute and subchronic effects on a marine crustacean, the calanoid copepod Acartia tonsa. Sublethal effects on A. tonsa larvae were investigated with a rapid and cost effective bioassay, which is based on the easily detectable morphological change from the last nauplius to the first copepodite stage during copepod larval development. The inhibition of larval development after 5 days exposure was a very sensitive endpoint, with 5-d-EC(50)-values as low as 0.026 mg/l (Tonalide(TM)), 0.059 mg/l (Galaxolide(TM)), 0.066 mg/l (inusk ketone) and 0.160 mg/l (Celestolide(TM)), respectively. These values were generally more than one order of magnitude below the 48-h-LC(50)-values found for adults, which were 0.47 mg/l (Galaxolide(TM)), 0.71 mg/l (Celestolide(TM)), 1.32 mg/l (musk ketone) and 2.5 mg/l (Tonalide(TM)). Since, the synthetic musks strongly inhibited larval development in A. tonsa at low nominal concentrations, they should be considered as very toxic. The larval development test with A. tonsa is able to provide important aquatic toxicity data for the evaluation of synthetic musks, for which there is little published ecotoxicological information available regarding Crustacea. It is suggested that subchronic and chronic copepod toxicity tests should be used more frequently for risk assessment of environmental pollutants. (C) 2002 Elsevier Science B.V. All rights reserved.