  We present a study of the vortex lattice in untwinned YBa$_2$Cu$_3$O$_{7-x}$
crystals, using a combination of muon spin rotation and neutron small angle
scattering measurements. Both methods show a very sharp melting temperature
consistent with a first order transition. The dependence of the melting
temperature on the angle of the field with respect to the crystallographic
c-axis is studied. The results are compared to thermal measurements.
