1. Gigantism has occurred in several different commercial varieties of tobacco, including Maryland types, Cuban, Connecticut Havana and Sumatra. It has also been associated with certain varietal crosses and species crosses. 2. Not only giant or mammoth types which breed true, but intermediate or hybria types occur spontaneously which subsequently give rise to a greater or less proportion of mammoth forms. 3. In crosses with normal varieties, the mammoth character is recessive, and F1 plants invariably biossom. The F1 plants average a somewhat higher leaf number than the normal parent which entered into the cross. 4. In the F2 generation mammoth plants occur in proportions approaching the theoretical ratio of 25 per cent. obtaining in a single Mendelian cross involving two contrasted unit characters.