Black duck (Anas rubripes) hens fed 10 ppm dry weight (approximately 3 ppm wet weight) of p,p'-DDE in the diet laid eggs with shells 22 percent thinner at the equator, 30 percent thinner at the cap, and 33 percent thinner at the apex than those of controls. Natural incubation increased shell cracking more than fourfold as compared with mechanical incubation. Hens removed cracked eggs from nests, and one hen terminated incubation. Hens fed DDE produced one-fifth as many ducklings as controls. DDE in eggs of dosed hens averaged 64.9 ppm wet weight.