SUMMARYTwo types of Polydactyly are described. Preaxial polydactylism is a relatively rare trait, involving a varying degree of duplication of the thumb. X-ray evidence is presented for this condition in a baby girl when first born and 5½ months later, showing the nature of its development. X-ray of the parents revealed that the mother was affected and the father was normal. Evidence points to the possibility that this trait follows a simple autosomaly type of inheritance. Postaxial polydactyly, the more common type, generally involves a duplication of the little finger (5th) with a varying degree of expression. A pedigree of a Negro family is described and X-ray evidence for this trait is given. Each duplicated miniature finger consists of two small but well formed phalanges. The trait seems to be transmitted as an incomplete dominant.