We report the results of a laboratory pedigree analysis describing the unique sex determining mechanism of the conchostracan shrimp, Eulimnadia texana. Natural populations of E. texana are mixtures of self-compatible hermaphrodites and males and represent one of the few known cases of androdioecy in animals. Hermaphrodites are of two types: amphigenic (producing both male and hermaphroditic offspring) and monogenic (producing only hermaphroditic offspring). We propose a simple genetic model to explain this polymorphism, and show by genetic analysis that males, amphigenics, and monogenics can be interpreted as three alternative phenotypes of a one-locus system of sex determination. We discuss the implications of this novel system of sex determination for understanding the evolution of reproductive systems.