Recently, P. Sarnak proposed to study the dynamical system determined by the arithmetic Moebius function $\mu:\N\to\{-1,0,1\}$. It has a natural factor determined by the characteristic function of the set of square-free numbers. This system has positive topological entropy and is not uniquely ergodic. The natural question is whether this system is intrinsically ergodic, i.e. has a unique invariant measure with maximal entropy. During the talk it will be shown that all B-free systems are intrinsically ergodic. The talk is based on a joint work with J. KuĊ‚aga-Przymus and B. Weiss.