THE synthesis of fullerenes encapsulating various metal atoms within the carbon cage (endohedral metallofullerenes) has stimulated wide interest(1,2) because of their unusual structural and electronic properties. Most of the metallofullerenes prepared so far have been based on C-82, and have incorporated lanthanum(1,3-5) yttrium(6,7), scandium(8) (10) and most of the lanthanide elements(11,12) Although there has been some debate about the endohedral nature of these compounds(2,13,14), observations using scanning tunnelling microscopy(15,16), extended X-ray absorption fine structure(17,18), transmission electron micrscopy(19) and electron spin; resonance(3,6) (8,10) have strongly suggested that the metal atoms are indeed inside the fullerene cages; theoretical calculations(20,21) also indicate that this is the case. But until now, no structural model has been derived experimentally to confirm the endohedral nature of the metallofullerenes. Here we report the results of a synchrotron X-ray powder diffraction study of Y@C-82 that confirms that the yttrium atom is located within the carbon cage. The yttrium atom is displaced from the centre of the C-82 molecule and is strongly bound to the carbon cage.