Inelastic neutron scattering spectra from Dy, Ho, Er or Tm ions dissolved in Mg show transitions that allow an accurate determination of the crystal-field strength, even at the low concentrations used (around 0.1 at.%). The values of the crystal-field parameters are comparable with those found for heavy rare-earth ions in other hexagonal metals. In most cases the transitions have a finite intrinsic line width which ranges from less than 0.04 to 0.34 MeV (FWHM) and is possibly due to exchange broadening.