  Polarisation-resolved Raman scattering measurements were performed on MgB_2
single crystals to determine the magnitude, symmetry and temperature dependence
of the superconducting gap. A single sharp peak due to Cooper pair breaking
appears in the electronic continuum below Tc, reaching an average maximum Raman
shift of 105 +/- 1 cm^{-1} (2 Delta(0) / k_{B}Tc = 3.96 +/- 0.09) and showing
up to 5 cm^{-1} anisotropy between polarised and depolarised spectra. The
temperature dependence of 2 Delta follows that predicted from BCS theory, while
the anisotropy decreases with decreasing temperature. It is concluded that the
Raman results are consistent with a slightly anisotropic s-wave gap in a
conventional BCS superconductor.
