The consequences of a simple type of exchange interaction between charge carriers in a broad energy band and localized magnetic moments will be discussed for a ferromagnetic semiconductor. The interaction causes (1) a splitting of the energy band into two bands for the different spin directions, (2) spin disorder scattering of the charge carriers. The calculated temperature and field dependence of the magnetoresistance are compared with experimental data of CdCr2Se4. The magnetoresistance of compoundsMeCr2S4,Me=Fe, Co, Cd was measured. Forn‐type CdCr2S4and forp‐type FeCr2S4andp‐type CoCr2S4, the magnetoresistance −&Dgr;&rgr;/&rgr;0was found to be 0.78, 0.05, and 0.05, respectively, at 12 kOe nearTc, whereasn‐type FeCr2S4and CoCr2S4show no effect. The magnetoresistance of the system Fe1−xCdxCr2S4was also measured. The temperature dependence of the polar magneto‐optical Kerr effect of CdCr2Se4is discussed in connection with the described model for the energy bands.