  We report on the detection with the Infrared Space Observatory (ISO) of a
molecular band at 57.5 microns (174 cm^{-1}) in carbon-rich evolved stars and
in Sgr B2. Taking into account the chemistry of these objects the most
likelihood carrier is a carbon chain. We tentatively assign the band to the
nu_5 bending mode of C_4 for which a wavenumber of 170-172.4 cm^{-1} has been
derived in matrix experiments (Withey et al. 1991). An alternate carrier might
be C_4H, although the frequency of its lowest energy vibrational bending mode,
nu_7, is poorly known (130-226 cm^{-1}). If the carrier is C_4, the derived
maximum abundance is nearly similar to that found for C_3 in the interstellar
and circumstellar media by Cernicharo, Goicoechea & Caux (2000). Hence,
tetra-atomic carbon could be one of the most abundant carbon chain molecules in
these media.
