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Hall resistivity and thermopower have been measured for the colossal magnetoresistance material Tl 2 Mn 2 O 7 over wide temperature and magnetic-field ranges. These measurements revealed that a small number of freeelectron-like carriers are responsible for the magnetotransport properties. In contrast to perovskite colossalmagnetoresistance ͑CMR͒ materials, the anomalous Hall effect is negligibly small even in the ferromagnetic state. An important property of Tl 2 Mn 2 O 7 is that the carrier density changes with temperature and magnetic field. The carrier density increases near T C 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.

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