Fusion of the elements, and alkali-metal halide when appropriate, in stoichiometric amounts in Ta containers followed by slow cooling results in high yields of the title compounds. X-ray structures refined for rhombohedral Cs8Ga11 (R (3) over bar c, Z = 6, a = 9.9962(5) Angstrom, c = 50.839(6) Angstrom) and Cs8Ga11Cl (R (3) over bar (3) over bar c, Z = 6, a = 10.0111(7) Angstrom, c 50.504(6) Angstrom) reveal isolated clusters of pentacapped, trigonal prismatic gallium anions, Ga-11(7-), the former compound being isostructural with K8In11 and A(8)Tl(11) (A = K, Rb, Cs). The clusters are arranged in pseudo-ccp layers separated by double layers of cesium atoms. The halide in Cs8Ga11Cl is bound in a preformed cavity within the cesium double layers where it is surrounded by eight cations. Of the nine examples reported for A(8)Tr(11)X, three chlorides occur in systems in which the binary A(8)Tr(11) do not form, Rb-Ga, Rb-In, and Cs-In. These halides are the first examples of Tr-11(7-) compounds that are valence phases and do not contain an extra alkali-metal cation and electron. Magnetic susceptibility data indicate an apparently localized electron in paramagnetic Cs-8-Ga-11 and diamagnetism for Cs8Ga11Cl.