The infrared absorption spectrum of a linear, 13-atom carbon cluster (C-13) has been observed by the use of a supersonic cluster beam-diode laser spectrometer. Seventy-six rovibrational transitions were measured near 1809 wave numbers and assigned to an antisymmetric stretching fundamental in the (1) Sigma(g)(+) ground state of C-13. This definitive structural characterization of a carbon cluster in the intermediate size range between C-10 and C-20 is in apparent conflict with theoretical calculations, which predict that clusters of this size should exist as planar monocyclic rings.