  We investigate the effect of $K^-$ condensed matter on bulk viscosity and
r-mode instability in neutron stars. The bulk viscosity coefficient due to the
non-leptonic process $n \rightleftharpoons p + K^-$ is studied here. In this
connection, equations of state are constructed within the framework of
relativistic field theoretical models where nucleon-nucleon and kaon-nucleon
interactions are mediated by the exchange of scalar and vector mesons. We find
that the bulk viscosity coefficient due to the non-leptonic weak process in the
condensate is suppressed by several orders of magnitude. Consequently, kaon
bulk viscosity may not damp the r-mode instability in neutron stars.
