Phally polymorphism, whereby some individuals in a hermaphroditic species have a complete functional penis (euphally), whereas others lack a penis (aphally) or have a reduced, non-functional penis (hemiphally), has evolved many times in pulmonate gastropods. Since the discovery of apophallation (penis amputation) in Ariolimax Mörch, 1860, aphallates in this genus have been attributed to apophallation. In laboratory studies in Ariolimax (Ariolimax) buttoni (Pilsbry and Vanatta, 1896), we found aphally in juveniles as well as in individuals reared in isolation to adulthood, demonstrating that the aphallate condition is not always due to apophallation. Some aphallate individuals reared in isolation from hatching produced eggs and viable hatchlings, providing the first demonstration of uniparental reproduction in this species. Egg-to-egg generation time in laboratory-reared individuals ranged from 10 months to more than 24 months. Anatomical data also elucidate the reproductive cycle of this species. Four reproductive states have been identified by the appearance of the reproductive system. Spring and early summer populations consist of individuals in the immature and intermediate reproductive states. The hypertrophied state was found from autumn until early spring. Egg-laying occurred in the laboratory in the fall and winter. Copulation consists of unilateral or simultaneously reciprocal intromissions and occurred in the laboratory between February and September. Very long copulations (more than 7 h) are more frequent than in other species of Ariolimax. Phally polymorphism, uniparental reproduction, and the variation in generation time should play important roles in determining the variance of mating success and the potential for sexual selection in this hermaphroditic species.