  We consider a time inhomogeneous strong Markov process $(\xi_t)_{t\ge 0}$
taking values in a Polish state space whose semigroup has a $T$-periodic
structure. We give simple conditions which imply ergodicity of the grid chain
$(\xi_{kT})_{k\in \mathbb{N}_0}$ In case of $d$-dimensional possibly degenerate
diffusions, the conditions are stated in terms of drift and diffusion
coefficient of the process.
