Total energy calculations with a local spin density functional have been applied to the Stone-Wales transformation in fullerene (C-60). In the formation of the almost exclusively observed I-h, isomer of C-60 with isolated pentagons, the final transformation must be from a C-2v isomer with two pentagon pairs. It was found that the energy barrier for this rearrangement was substantially reduced in the presence of an extra carbon atom. Such atoms were found to bind loosely, preferentially to regions in which there were paired pentagons. Pentagon rearrangements, which are necessary steps in the growth of fullerenes, may therefore result from autocatalysis by carbon.