Molecular species containing metal-metal bonds have been known for a long time(1). But these have generally involved transition metals; metal-metal bonds between main-group elements were discovered only more recently(2). Within the past decade, polyhedral Al-4, Al-12 and Ga-4 species have all been characterized by X-ray diffraction(3,4), and the chemistry of low-valent polynuclear Al species in particular has nourished owing to the development of condensation techniques for preparing them by disproportionation of aluminium monohalides(4). By using this approach, we now report the identification and characterization of an Al-77 cluster, which is dramatically larger than any seen hitherto and can be considered an intermediate species to the formation of the bulk metal, X-ray diffraction shows that the cluster has a central Al atom surrounded by three concentric polyhedral shells containing 12, 44 and 20 Al atoms. This Al-77 core is stabilized by 20 organic ligands, preventing the formation of the bulk metal, Studies of such clusters should provide insights into the crossover between molecular species and the bulk metal for main-group elements.