The abundance, diet, and prey relationships of American mink Mustela vison were studied in the Słońsk Reserve (W Poland) on two plots: shore and reservoir. Estimated mink number within the Reserve was 69 in autumn–winter 1998/1999 and 50 during spring 1999. The diet of American mink consisted mainly of mammals, birds and fish. In autumn–winter, birds formed 4–16%, whereas mammals constituted up to 56% and fish up to 62% of the biomass consumed, depending on the plot. In spring and summer, however, birds formed 45–60% of the biomass consumed in the reservoir and 35–46% of the biomass taken by mink on the shore. The European coot Fulica atra was the most frequently consumed prey. In spring, mink removed 7.8% (N=278 killed birds) of coots nesting in the Reserve, 1.8% (N=9) of breeding grebes Podiceps spp. and 11.2% of ducks (N=93 taken birds). On straw platforms only 13.6% of greylag geese Anser anser broods were successful. From 35 to 77% of the nests on straw platforms were destroyed by mink. In wooden boxes 46.4% of mallard Anas platyrhynchos and 33.3% of shelduck Tadorna tadorna nests were successful. American mink destroyed 22–40% of the nests in boxes. However, the arrival of American mink to the Słońsk Reserve has not resulted in a noteworthy decrease in waterfowl populations.