The electrical conductivity and Hall coefficient of a variety of single crystals of zinc selenide have been measured. As-grown, undoped crystals have a resistivity ( approximately 10¹² Omega cm) which is too large for successful Hall measurements to be made. However, after heat treatment at 850 degrees C in molten zinc, the resistivity of a sample is reduced to about 10^-1 Omega cm. The samples are fairly heavily compensated. The Hall mobility of the free electrons is about 530 cm² V^-1 s^-1 at room temperature when optical mode scattering is the dominant mobility-limiting mechanism. At lower temperatures, ionized impurity scattering becomes dominant. Crystals containing grown-in impurity donors such as indium, gallium and chlorine have also been investigated, as have crystals containing aluminium. Increasing the concentration of aluminium leads to overlap of the shallow donor wavefunctions with consequent impurity banding and the production of a degenerate semiconductor. Increasing the concentration of indium, gallium or chlorine is accompanied by anomalous behaviour.