This paper reports the determination of the X-ray molecular structure of 4-deoxy-3'-bromopyrido[1',2'-1,2]imidazo[5,4-c]rifamycin S, carried out in order to unequivocally define the general structure of a new series of rifamycin SV derivatives, which are potent antibacterial agents, and are not absorbed at the gastroenteric level. They have been prepared by Alfa Farmaceutici, Bologna, by condensing 2-aminopyridine derivatives to 3-bromorifamycin S. The solid state X-ray study has confirmed the structure proposed on the basis of 1H NMR studies in solution. It has also shown that the newly formed pyridoimidazo system is in a mesomeric betaine form, the pyrido nitrogen being positively charged and the imidazo nitrogen being negatively charged. This feature is believed responsible for the pharmacokinetic behavior of these new drugs, one of which, denoted either as rifamycin L 105 or rifaximin, is actually under clinical trial as a topical intestinal disinfectant.