The morphology, histology and histochemistry of the reproductive system of Arion ater rufus (Gastropoda, Pulmonata) have been described. Contrary to previous opinion, the loop of the hermaphrodite duct has been found not to be a vesicula seminalis. Instead, the distal convoluted part of the duct acts as a true seminal vesicle. The changes which occur in the reproductive system during the life of the animal, have been examined in detail. Increasing age has a marked effect upon the development of the reproductive organs. It has been found that any of the epithelial cells of the acinar wall of ductules in the hermaphrodite gland may transform into a follicular cell. The nutritive function of nurse cells with relation to the development of spermatozoa is discussed. Fluctuations in the number of oocytes in the hermaphrodite gland have been determined and its significance in relation to the development of spermatozoa is discussed. Arion ater rufus is protandric. During the first month of life, each individual is male; from two months onwards it is a true hermaphrodite, till finally it becomes female at the age between five to twelve months. Histological observations suggest that sex determination in the gametes is the result of an inherent capacity of the acinar epithelial cells to respond in different ways to an inducing mechanism. The distribution within the reproductive system of alkaline phosphatase, lipids, glycogen and other polysaccharides has been described. The significance of the distribution and quantitative changes of these materials is discussed.