The anisotropic E.S.R. spectra of SiH3X- and SiD3X- radical anions (X=Cl, Br, I) have been observed after γ-irradiation of solid solutions containing 5 mol per cent of SiH3X or SiD3X in tetramethylsilane. The spectra were analysed by a comparison of the spectra of SiH3X- with those of SiD3X-. As a result, the observed spectra of SiH3X- were assigned mainly to anisotropic hyperfine structures of a halogen and a hydrogen. Moreover, as to SiH3Cl-, hyperfine structures of the other two hydrogens with small couplings and the 29Si isotope with a large coupling were detected. On the basis of the spectral analyses, it was concluded that, in contrast with the CF3X- radical anions (X=Cl, Br, I) which have C 3v symmetry, the SiH3X- radical anions have a trigonal bipyramid structure with a halogen and a hydrogen in the axial positions and two hydrogens in the equatorial positions, as for SiF3Cl- and phosphoranyl radicals.