  Despite extensive research on the skutterudites for the last decade, their
electric crystalline field ground state is still a matter of controversy. We
show that Electron Spin Resonance (ESR) measurements can determine the full set
of crystal field parameters (CFPs) for the Th cubic symmetry (Im3) of the
Ce$_{1-x}$R$_{x}$Fe$_{4}$P$_{12}$ (R = Dy, Er, Yb, $x\lesssim 0.003$)
skutterudite compounds. From the analysis of the ESR data the three CFPs, B4c,
B6c and B6t were determined for each of these rare-earths at the Ce$^{3+}$
site. The field and temperature dependence of the measured magnetization for
the doped crystals are in excellent agreement with the one predicted by the
CFPs Bnm derived from ESR.
