Experimental evidence is presented for the existence of a current-driv
en plasma instability in superconducting films. Anomalous dissipation
has been observed in a current-carrying superconducting YBa2Cu3O7 micr
obridge film which is separated from a superconducting tin film by an
insulating spacer layer. This is consistent with the instability predi
cted for a system of two superconductors separated by an ideal interfa
ce and carrying oppositely directed currents. New calculations show th
at in spite of the presence of the insulting layer, the instability sh
ould still occur due to proximity effects.