Sergei Kuksin, Armen Shirikyan On a Ruelle-Perron-Frobenius type theorem (84K, Postscript) ABSTRACT. We consider the problem of uniqueness of a stationary measure for Markov semigroups on a Polish space. Assuming that the transition function is ``uniformly Feller'' for a family of functions $R$ and uniformly irreducible, we show that any two measures coincide on $R$. In particular, if $R$ is a determining family, then the above conditions ensure the uniqueness of stationary measure. The result obtained has applications in the ergodic theory of randomly forced PDE's.