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The growth of single crystals corresponding to x=0 (high-purity magnetite), x = 0.01 and x = 0.04 in the composition series CoxFe3-xO4 is described. First- and second-order anisotropy constants of these crystals are determined by the torque method over the temperature range 120Â¿450Â°K. The substitution of small amounts of cobalt for iron is found to add a positive contribution to K1 and a negative contribution to K2. At low cobalt concentrations the magnitude of this contribution is a linear function of the amount of cobalt added. It is suggested that the effect of cobalt on the anisotropy is enhanced by the presence of divalent iron in magnetite, and it is concluded that the change in sign of K in magnetite at about 130Â°K is not caused by the presence of cobalt or any other foreign impurity.