Ochratoxin A (OA), a potent nephrotoxic mycotoxin, was fed at 2 ppm in the diet to Khaki Campbell ducklings from hatch to 18 days old. It caused retarded growth, enlargement of the kidneys and liver and regression of the thymus. Light and electron microscopical changes seen in the liver included an accumulation of glycogen and the presence of mis-shapen mitochondria in the hepatocytes. OA caused thickening of the glomerular basement membrane and in infiltration of lymphoid cells into the kidney. There was an increased deposition of IgG in the glomeruli. OA-fed ducklings had normal levels of circulating IgG in their sera and there was a subepithelial migration of IgG-positive cells in the bursa of Fabricius. In general, OA-induced changes seen in ducklings, both with the light and electron microscopes, were similar to, but less pronounced than those seen in the fowl and turkey.