We report on resonance-enhanced multiphoton electron detachment (REMPED) spectra of C-10(-) and C-11(-). Both species were obtained by laser evaporation of graphite in a flow of helium. The spectra obtained under several different evaporation conditions suggest that both anions occur as well in a ring as in a chain form. The ring form dominates at low laser fluences. For the species C-10(-) our data indicate a third isomeric form which may consist of a somehow distorted ring structure.