Abstract. C39H49NOI3.CH3OH.H2 O, M r = 789.87, monoclinic, P21, Z= 2, F(000)= 844, MoK~t radiation, 2=0.71069A, room temperature; a= 11.860(4), b=9.140(3), c=20.423(4) A, fl= 90.72(2) ° , U--2213(1)A 3, Dx=l.18Mgm -3, g(Mo K~t) = 0.099 mm -l, R = 0.071, wR = 0.099 for 4043 observed reflections. The structure was solved by direct methods, using one-phase and two-phase semiinvariants in an active way. An a posteriori examination of the convergence/divergence map showed that conventional direct methods failed because of the role played by aberrant triplet relationships in the very early stages of the phase-extension procedure. The conformational stability of the ansa-bridge, as observed in many active rifamycin derivatives, is unaffected in this new derivative by the acetylation at O(10) and the reduction at C(ll). Nevertheless the acetylation at O(10) does prevent biological activity.