In the mangnese-zinc-ferrous ferrite system (Mnx−yZnyFe3−xO4), which is of considerable technical importance, the magnetocrystalline anisotropy cannot be measured directly because ferrite single crystals containing zinc cannot be produced. It is shown in the paper that the sign and, to some extent, the magnitude of the first-order anisotropy constant,K1can be determined by measuring the magnetostriction of polycrystalline samples as a function of the applied field. This is possible if the magnetostriction constants λ100and λ111have different signs, which is the case in the above system. The result is that in some of the compositions the anisotropy constantK1passes through zero at around room temperature. This explains the occurrence of a secondary peak in the permeability/temperature curve of these ferrites. As the saturation magnetostriction in the same composition is very small, this represents the oxidic analogue of Permalloy.