  We have recorded the Doppler profile of a well-isolated rovibrational line in
the \nu2 band of 14NH3. Ammonia gas was placed in an absorption cell
thermalized by a water-ice bath. By extrapolating to zero pressure, we have
deduced the Doppler width which gives a first measurement of the Boltzmann
constant, kB, by laser spectroscopy. A relative uncertainty of 2x10-4 has been
obtained. The present determination should be significantly improved in the
near future and contribute to a new definition of the kelvin.
