Abstract It has been realized in the past years that high solubilities of various transition-metal impurities in II–VI compounds offer novel opportunities to study the fundamental aspects of transport phenomena in semiconductors. In the case of Mn-based compounds doped with hydrogenic-like impurities, a strong sp-d exchange interaction was found to affect dramatically the low-temperature resistance and magnetoresistance in the vicinity of the metal-to-insulator transition. These effects turned out to reflect the influence of the giant exchange spin-splitting on quantum corrections to the conductivity of a disordered Fermi liquid. Furthermore, investigations of the magnetic field-induced insulator-to-metal transition yielded valuable information on the critical behavior and driving mechanisms of the transition. Studies of Fe-based compounds, particularly those of Hg 1- x Fe x Se, shed new light on the old problem of how can transport properties be affected by resonant states. High electron mobilities, which were detected in the material in the mixed-valence region of Fe ions, were shown to be a consequence of the domination of inter-site Coulomb interactions over effects of sp-d hybridization. These two aspects of semimagnetic semiconductors are briefly described. Additionally, the problems of transport along grain boundaries in Hg 1- x Mn x Te and of superconductivity in HgSe: Fe are mentioned.