Cryst. polymorphous forms of the rifaximin (INN) antibiotic named rifaximin  and rifaximin  useful in the prodn. of medicinal prepns. contg. rifaximin for oral and topical use and obtained by means of a crystn. process carried out by hot-dissolving the raw rifaximin in Et alc. and by causing the crystn. of the product by addn. of water at a determinate temp. and for a determinate period of time, followed by a drying carried out under controlled conditions until reaching a settled water content in the end product, are the object of the invention
