Iodinated casein at 220 mg. kg. of feed was fed to Muscovy ducklings to determine its effect on sexually differentiated growth. Nine male and eight female treated ducklings averaged 2,135 and 1,520 g. at eight weeks as compared to 2,315 and 1,588 g. for the untreated nine males and eight females. Feed conversion was 2.99 for treated and 3.60 for untreated ducklings.