The crystal structure of aclacinomycin oxidoreductase (AknOx), a tailoring enzyme involved in the biosynthesis of the polyketide antibiotic aclacinomycin, was determined to 1.65 A resolution by multiwavelength anomalous diffraction using data from selenomethionine-substituted crystals. The crystals belong to space group P2(1), with unit-cell parameters a = 68.2, b = 264.5, c = 68.2 A, beta = 119 degrees . Analysis of the intensity statistics clearly showed the presence of pseudomerohedral twinning. The data set could also be indexed and scaled with an R(sym) of 0.072 in the orthorhombic space group C222(1) (unit-cell parameters a = 69.7, b = 117.5, c = 264.4 A), indicating the possibility of pseudomerohedral twinning along the diagonal between the monoclinic a and c directions. Refinement using this twin operator resulted in an R(free) of 24.2%. A monoclinic lattice with a = c and beta close to 120 degrees can emulate a hexagonal metric, with the possibility of a threefold twin operator along the b axis and three twin domains. Refinement assuming three-domain twinning gave a final R(free) of 26.5%. The structure of AknOx can be thus refined with comparable R(free) values using either of the twin operators separately, suggesting the possibility that crystals of AknOx contain six twin domains generated by the twofold and threefold twin operators perpendicular to each other. Both twin operators coincide with noncrystallographic symmetry axes that may promote twinning.