SINCE the discovery of fullerenes(1), efforts have been made to trap metal atoms inside fullerene cages(2), and both endohedral(3,4) and exohedral(5,6) metallofullerenes have been synthesized. There is, however, a third possibility : a ’networked’ metallofullerene, where the metal atom is incorporated into the carbon cage. Here we report the results of experiments to study the structure and reactivity of gas-phase fullerenes doped with niobium (NbCn+ with n = 28-50). These experiments, which use injected-ion drift-tube techniques, indicate that for fullerenes containing an even number of carbon atoms the metal is endohedral, but for fullerenes with an odd number of carbon atoms, the niobium metal is bound as a part of the carbon cage. Thus, networked metallofullerenes appear to be a stable class of metallofullerene. We suggest that such metallofullerenes can form if the metal atom retains sufficient electron density to form several strong covalent metal-carbon bonds.