The magnetization curves of CaMnO 3−␦ ͑␦ = 0.20 and 0.25͒ single crystals in pulsed magnetic fields up to 350 kOe were studied, and the threshold increase of magnetization at H cr ϳ 50 kOe and T = 4.2 K was discovered. This anomaly is explained by melting of the charge ordering, which is associated with oxygen vacancy ordering. The jump at the metamagneticlike transition is equal to 0.02 B / Mn; no hysteresis is evidenced in the field-up and field-down processes. The magnetization dependence versus magnetic field is almost linear at T = 77 K. The results obtained are explained assuming high stability of the antiferromagnetic G-type charge ordering phase at T = 4.2 K and its destabilization under magnetic field or temperature increase because of the double-exchange reinforcement between Mn 3+ and Mn 4+ ions. The oxygen vacancy ordering can be considered as an additional degree of freedom in manganites.The unusual physical properties of perovskite manganites are caused by the strong interplay between charge, spin, lattice, and orbital degrees of freedom. The long-range charge ordering was observed in R 1−x Ca x MnO 3 ͑R = La, Pr, Nd, and Sm͒ systems at concentrations x =1/ 2, 2 / 3, and 3 / 4 ͑see Refs. 1 and 2, and references therein͒. The charge ordering is destroyed ͑"melted"͒ under the influence of strong magnetic field. 3 At the same time, the metamagnetic transition from the antiferromagnetic ͑AFM͒ charge-ordered ͑CO͒ state to the ferromagnetic ͑FM͒ metallic state occurs. The metamagnetic transition associated with melting of the orbital ordering was observed in Sm 0.8 Ca 0.2 MnO 3 . 4 Several years ago, using powder electron diffraction, 5 it was shown that the oxygen vacancy ordering takes place in CaMnO 3−␦ polycrystals at ␦ = 0.2, 0.25, 0.333, and 0.5. Recently, the ordering of the oxygen vacancies was revealed in CaMnO 3−␦ single crystals grown by the floating-zone method. 6 The oxygen vacancy ordering corresponding to ␦ = 0.2 and 0.25 was established in these crystals by the method of thermal neutron diffraction. 7,8 The superstructure reflections were observed up to a temperature of 840 K. The charge ordering is connected with the oxygen vacancy ordering and persists in a wide temperature range. The ground state of CaMnO 2.75 is of AFM G type. 8 In the composition CaMnO 2.75 , as well as in La 0.5 Ca 0.5 MnO 3 , 9 the numbers of Mn 4+ and Mn 3+ ions are equal, however, genesis of the charge ordering and the type of the AFM CO state differ. Therefore, one could say that in nonstoichiometic CaMnO 3−␦ manganites, there is an additional degree of freedom, namely, the oxygen vacancy ordering. La 0.5 Ca 0.5 MnO 3+␦ manganite is a metamagnet, it undergoes the transition from the CE-type AFM state to FM state under magnetic field at various temperatures below the charge ordering temperature T CO ϳ 190 K. 3 The charge ordering in the G-type AFM phase and magnetization data of CaMnO 3−␦ at high magnetic fields were not studied earlier. The goal of the present work is to determine the magnetization peculiarities ...