  Low-ionization-state far-infrared (FIR) emission lines may be useful
diagnostics of star-formation activity in young galaxies, and at high redshift
may be detectable from the ground. In practice, however, very little is known
concerning how strong such line emission might be in the early Universe. We
attempted to detect the 158 micron [C II] line from a lensed galaxy at z =
4.926 using the Caltech Submillimeter Observatory. This source is an ordinary
galaxy, in the sense that it shows high but not extreme star formation, but
lensing makes it visible. Our analysis includes a careful consideration of the
calibrations and weighting of the individual scans. We find only modest
improvement over the simpler reduction methods, however, and the final spectrum
remains dominated by systematic baseline ripple effects. We obtain a 95 per
cent confidence upper limit of 33 mJy for a 200 km/s full width at half maximum
line, corresponding to an unlensed luminosity of 1x10^9 L_sun for a standard
cosmology. Combining this with a marginal detection of the continuum emission
using the James Clerk Maxwell Telescope, we derive an upper limit of 0.4 per
cent for the ratio of L_CII/L_FIR in this object.
