Hall resistivity and thermopower have been measured for the colossal magnetoresistance material Tl2Mn2O7 over wide temperature and magnetic-field ranges. These measurements revealed that a small number of free-electron-like carriers are responsible for the magnetotransport properties. In contrast to perovskite colossal-magnetoresistance (CMR) materials, the anomalous Hall effect is negligibly small even in the ferromagnetic state. An important property of Tl2Mn2O7 is that the carrier density changes with temperature and magnetic field. The carrier density increases near Tc as the temperature is lowered or as the magnetic field is increased, which explains the CMR of this material. The conduction-band edge shift caused by the development of magnetization is a possible mechanism causing the carrier density change.