Observations of the free-surface behavior under shock compression by the gapped-flat mirror method were performed onB4CandSi3N4ceramics to study their shock-yielding properties. Jagged profiles of the moving free-surface in the plastic region, with a special scale of about one mm and a maximum local displacement of a few 10s of &mgr;m, were observed forB4Cpolycrystals. Similar profiles forSi3N4polycrystals were smooth. Such profiles forB4Cpolycrystals were also observed in the elastic region. It is suggested that these observations reflect the heterogeneous nature of shock compression in solids, and further indicate that a macroscopic slip system plays an important role in the elastoplastic transition ofB4Cmaterial under shock compression and decompression. ©1997 American Institute of Physics.