Let be an open subset of R d and H = − d i, j=1 ∂ i c i j ∂ j be a second-order partial differential operator on L 2 ( ) with domain C ∞ c ( ), where the coefficients c i j ∈ W 1,∞ ( ) are real symmetric and C = (c i j ) is a strictly positive-definite matrix over . In particular, H is locally strongly elliptic. We analyze the submarkovian extensions of H , i.e., the self-adjoint extensions that generate submarkovian semigroups. Our main result states that H is Markov unique, i.e., it has a unique submarkovian extension, if and only if cap (∂ ) = 0 where cap (∂ ) is the capacity of the boundary of measured with respect to H . The second main result shows that Markov uniqueness of H is equivalent to the semigroup generated by the Friedrichs extension of H being conservative.