The NaCl-type compounds Eu1-xGdxSe form a continuous series of solid solutions between the ferromagnetic insulator EuSe and the antiferromagnetic metal GdSe. We have studied the temperature dependence of electrical resistivity ρ, in Eu1-xGdxSe single crystals and polycrystals between 2.5°and 300°K; we find, in qualitative agreement with earlier observations, for small x, an exponential increase in ρ similar to normal semiconductors but a very sharp drop in ρ below the magnetic transition temperature. The resulting peak in ρ decreases sharply with applied magnetic field strength and increasing Gd concentration. The temperature dependence of the Hall effect in single-crystal samples of Eu 0.95Gd0.05Se indicates as origin of the resistivity anomaly an unusually large effect of the magnetic order on the carrier mobility. Preliminary thermoelectric power measurements support this conclusion. The possibilities of describing strong interactions between conduction electrons and localized spins by scattering theory and by magnetic self-trapping of electrons in the spin lattice are discussed. © 1967 The American Institute of Physics.