Environmental fate and effect of ZPT, a booster biocide in anti-fouling paints for boats has been studied. Using a new analytical method for simultaneous determination of zinc- and copper pyrithione (ZPT/CPT) showed that up to 50% of added nominal ZPT was tranchelated to CPT at ambient seawater copper concentrations. Both compounds are photodegradable, with half-lives between 7 to 45 minutes depending on light intensities. At lower light intensities, i.e. in natural waters containing organic matter, and in the dark, the compounds are stable for >,48 hours. The toxicity of ZPT and CPT varied between EC50-values of 1.6-60 nM for pelagic bacteria, algae and zooplankton communities. ZPT and CPT affected the benthic community nutrient cycling at concentrations over 0.001 nmol/g dry sediment, especially cycling of nitrogen. Since ZPT in boat-paint will be tranchelated to the more stable, and for the pelagic community more toxic compound CPT when released into the marine environment, CPT should be included in future risk assessment of ZPT.