Nonexponential photon echo decay is observed in several hosts containing Er3+ in high magnetic fields. Concepts extended from electron spin resonance studies of spectral evolution of the coherently prepared ensemble due to fluctuating fields produced by electron and nuclear spin flips of the surrounding ions are used to describe the echo behavior. At low dopant levels and in hosts where the Er3+ have a large ground state g-value such that the upper electron spin state is depopulated, optical dephasing is dominated by the dynamics of the F- nuclear spins which produce highly nonexponential decay due to the melting of the frozen core. At higher dopant levels and where the upper Er3+ spin state is highly occupied, electron spin-electron spin interactions control the dephasing leading to more nearly exponential echo decay behavior. © 1991.