A thallium-manganese pyrochlore, Tl 2 Mn 2 O 7 , shows magnetotransport properties including a colossal magnetoresistance (CMR) quite similar to those observed in the hole-doped perovskite manganese oxides. The pyrochlore material, however, differs both structurally and electronically from the perovskites. Neither Zener's double-exchange nor the phonon-mediated mechanism, which is usually applied to the perovskite compounds, seems to explain the CMR in the pyrochlore. A distinctive electronic band structure, in which a hybridized band of Tl(6s)-O(2p)-Mn(3d) plays an essential role in magnetic and transport properties, is described. The CMR mechanism for Tl 2 Mn 2 O 7 is discussed in terms of a drastic change in the electronic band structure.