The crystal structure of the p-iodoanilide derivative of rifamycin Y, at room temperature, has been determined by three-dimensional methods. There are two formula units in the monoclinic unit cell, a = 9.49, b = 18.96 , c = 15.30 Å, $\beta$ = 90°48', with space group P21. The constitution and configuration of rifamycin Y, C₃₉H₄₇NO₁₅, as well as many of its conformational details, are therefore now established through these crystallographic results obtained from the study of its p-iodoanilide derivative.