We report the isolation and characterization of the bisfullerene C-121, the first all-carbon molecule to contain a homofullerene (also called a fulleroid) cage. This unsymmetrical isomer of C-121,along with a symmetrical isomer of C121 and C-122, were obtained by thermolysis of a mixture of C60CBr2 and C-60 and separated by high performance liquid chromatography. The predominant isomer of C-121 has a spiro carbon atom bridge that connects to one of the cages through an open [5.6] ring junction (i.e., between a pentagon and a hexagon) and to the other through a closed [6.6] ring junction. Ab initio calculations indicate that the unsymmetrical structure is more stable than either of the symmetrical alternatives, with the bridging carbon atom attached to both C-60 cages through closed [6.6] ring junctions or attached to each cage through open [5.6] ring junctions. Experimental evidence for the unsymmetrical structure comes from the C-13 NMR and UV/vis spectra. Electrochemical reduction of this bisfullerene shows three pairs of distinct, reversible peaks that correspond to each of the first three reductions of the [60]fullerene cages. This is consistent with the presence of a homofullerene unit and shows the similarity of the redox behavior to that of C-60 The first reduction potential of C-121 is slightly shifted toward more positive values than that of [60]fullerene.