Fully C-13-enriched C-60 has been investigated by pulsed electron-nuclear double-resonance (ENDOR) spectroscopy at W-band frequency at 1.2 K, For molecular C-60 the ENDOR spectra reflect the distortion of the molecule upon triplet excitation, and the fine-structure parameter D is found to be negative. For a single crystal of C-60 the ENDOR spectra characterize the triplet excitation as delocalized over a pair of neighboring C-60 molecules. (C) 1998 Academic Press.