The dephasing time of coherently prepared Pr3+ impurity ions in a La F3 crystal is measured by optical free-induction decay. Above 4°K, phonon processes severely shorten the decay time. Below 4°K, we observe a temperature-independent decay time of 0.38 μsec. This is surprisingly short considering that there are no contributions from first-order hyperfine, phonon, or Pr3+-Pr3+ interactions. © 1976 The American Physical Society.