This paper considers supervisory control of probabilistic discrete event systems (PDESs). PDESs are modeled as generators of probabilistic languages. The supervisory control problem considered is to find, if possible, a supervisor under whose control the behavior of a plant is identical to a given probabilistic specification. The probabilistic supervisors we employ are a generalization of the deterministic ones previously employed in the literature. At any state, the supervisor enables/disables events with certain probabilities. Necessary and sufficient conditions for the existence of such a supervisor, and an algorithm for its computation are presented.