The ground state hyperfine resonances of the C4v centre of Ho3+ in CaF
2 have been detected by using a hole burning microwave-optical double-
resonance technique. Resonances are detected, in zero magnetic field,
at 2.250, 1.608 and 0.842 GHz corresponding to the transitions between
the hyperfine levels of the ground electronic state. In addition, wea
ker signals are observed at 2.340, 1.669 and 0.872 GHz. These resonanc
es correspond to the hyperfine transitions within the neighbouring ele
ctronic state 1.7 cm-1 higher in energy, and result from thermally all
owed electronic transitions between hyperfine levels with the same nuc
lear spin projection. The low field Zeeman behaviour of these resonanc
es has also been studied and updated values for the hyperfine and crys
tal field splitting parameters are reported.