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The reaction of trimethylgallium or trimethylindium with 2,3-dicarba-nido-hexaborane(8) yields the closed polyhedral species 1-CH3GaC2B4H6 and 1-CH3InC2B4H6, respectively, in which the metal atom in each case forms one apex of a pentagonal bipyramidal cage. These carboranes have been structurally characterized by boron-11 and proton NMR, mass spectra, and infrared spectra, and by a single crystal X-ray diffraction study of the gallium compound. The X-ray results have disclosed two distinctive features: The gallium atom is closer to the boron atoms than to the carbon atoms in the five-membered equatorial ring; The Ga-CH3 axis is tilted by 23 deg with respect to the perpendicular to the equatorial plane. These findings are interpreted in terms of a qualitative molecular orbital model. The gallium and indium compounds are thermally stable to at least 100C but pyrolysis at high temperatures generates 2,3-C2B4H8. Bromine attacks both metallocarboranes at room temperature, destroying the cage structure and forming the dimer of (CH3)2GaBr and, presumably, the indium analog respectively. (Author)