  We report novel non-linear phenomena in the $ab$-plane microwave impedance of
$YBaCu_{2}O_{7-\delta }$ single crystals. The $R_s$ vs. $H_{rf}$ data are well
described by the non-linear RSJ model : $\dot{\phi}+\sin \phi =i_{rf}\cos
\omega t$. The entire crystal behaves like a single Josephson junction. The
extraordinary coherence of the data suggests an intrinsic mechanism.
