Fungal, bacterial and malarial infections, as well as malnutrition caused heavy mortality in a group of wild-trapped canvasbacks (Aythya valisineria) held in 10 X 3 X 2 m open-water pens. Deaths occurred between 21 and 158 days after confinement and were associated with infections of Aspergillus fumigatus, Escherichia coli, Staphylococcus sp., Streptococcus sp., Klebsiella sp., Enterobacter sp., and Plasmodium sp. Infection and mortality was believed to result from reduced resistance associated with confinement. Fourteen canvasbacks released onto large ponds survived throughout the period during which the penned birds died.