Regular series of microwave absorption lines, observed in the weakest junctions of superconducting single crystals and irregular particles, result from the nucleation of fluxons. This nucleation has a threshold microwave field Hc1J, above which the absorption lines broaden into bands of width H1-Hc1J. The threshold microwave field is anisotropic with respect to the orientation of the junction and decreases with increasing temperature, going to zero at Tc. Fluxon nucleation in Josephson junctions may be the major source of RF loss in zero magnetic field at high power levels. © 1989.