  Density functional calculations are used to study magnetic and electronic
properties of the spinel ferrites, ZnFe$_2$O$_4$ and MnFe$_2$O$_4$. Correct
magnetic orderings are obtained. ZnFe$_2$O$_4$ is predicted to be a small gap
insulator in agreement with experiment. MnFe$_2$O$_4$ is found to be a low
carrier density half-metal in the fully ordered state. However, strong effects
on the electronic structure are found upon partial interchange of Fe and Mn
atoms. This indicates that the insulating character may be due to Anderson
localization associated with the intersite Mn-Fe disorder.
