Crystalline polymorphous forms of the rifaximin (INN) antibiotic named rifaximin α and rifaximin β, and a poorly crystalline form named rifaximin γ have been discovered. These forms are useful in the production of medicinal preparations for oral and topical use and can be obtained by means of a crystallization process carried out by hot-dissolving the raw rifaximin in ethyl alcohol and by causing the crystallization of the product by the addition of water at a determinate temperature and for a determinate period of time. The crystallization is followed by drying carried out under controlled conditions until a specific water content is reached in the end product.