The effects of the strobilurin fungicide azoxystrobin were studied in rackish water microcosms, with natural plankton communities and ediment. Two experiments were conducted: Experiment 1 (nominal conc. 0, 5 and 60 mu g/L, 24-L outdoor microcosms for 21 days) and a second, ollow-up, Experiment 2 (nominal conc. 0, 3, 7.5, 15 mu g/L, 4-L indoor icrocosms for 12 days). The microcosms represent a simplified brackish ater community found in shallow semi-enclosed coastal areas in gricultural districts in the Baltic Sea region. Measured water oncentrations of the fungicide (Experiment 1) were, on average, 83 and 2% of nominal concentrations directly after application, and 25 and 30% fter 21 days, for the low and high dose treatments, respectively, orresponding to mean DT50-values of 15.1 and 25.8 days, for low and igh dose treatments, respectively. In Experiment 1, direct toxic ffects on calanoid copepods at both test concentrations were observed. imilarly, in Experiment 2, the copepod abundance was significantly educed at all tested concentrations. There were also significant econdary effects on zooplankton and phytoplankton community structure, tanding stocks and primary production. Very few ecotoxicological tudies have investigated effects of plant protection products on Baltic rganisms in general and effects on community structure and function pecifically. Our results show that azoxystrobin is toxic to brackish ater copepods at considerably lower concentrations than previously eported from single species tests on freshwater crustaceans, and that irect toxic effects on this ecologically important group may lead to ascade effects altering lower food webs and ecosystem functioning.