Abstract.The Ruelle zeta-function of the geodesic flow on the sphere bundle S(X) of an even-dimensional compact locally symmetric space X of rank 1 is a meromorphic function in the complex plane that satisfies a functional equation relating its values in s and -s . The multiplicity of its singularity in the central critical point 5 = 0 only depends on the hyperbolic structure of the flow and can be calculated by integrating a secondary characteristic class canonically associated to the flow-invariant foliations of S(X) for which a representing differential form is given.Let Y = G/K be a rank one symmetric space of the non-compact type, i.e., Y is a real, complex, or quaternionic hyperbolic space or the ( 16-dimensional) hyperbolic Cayley-plane. Let T be a uniform lattice in the (connected simple) isometry group G of y without torsion. Y acts properly discontinuous on Y = G/K (K a maximal compact subgroup of G), and X = Y\G/K is a compact locally symmetric space. We consider I asa Riemannian manifold with respect to an arbitrary (constant) multiple g of the metric go induced by the Killing form on the Lie algebra of G. Then the Riemannian manifold (X, g) is a space of negative curvature.The negativity of the curvature of the metric g on X implies the existence of an infinite countable set of prime closed geodesies in X with a discrete set of prime periods accumulating at infinity.Let , be the geodesic flow on the unit sphere bundle S(X) of the space (X,g).The prime period of a periodic orbit of O, on S(X) coincides with the length of the closed geodesic in X obtained by projecting the periodic orbit into X. Now we use these periods to define the zeta-function