Experiments on the growth of CuO single crystals by crystallization from flux in the CuO–Bi2O3– PbO–PbF2, CuO–Bi2O3–Li2O, CuO–Bi2O3–B2O3, CuO–BaO–Y2O3, and CuO–MOx systems (M = P, V, or Mo) have been performed. The best results were obtained in crystallization in the CuO–Bi2O3–PbF2 system: prismatic single crystals of platelet- and needlelike or isometric habit with dimensions up to 1 × 10 × 10, 1 × 1 × 20, or 6 × 6 × 8 mm, respectively, have been grown. The CuO crystals show polysynthetic twinning in the form of numerous alternating light and dark bands bound by systems of parallel straight lines on the {110} and {111} faces. A possible model of twinning associated with the Cu2O CuO transformation is considered.