A 3-dimensional Riemannian manifold is called Killing submersion if it admits a Riemannian submersion over a surface such that its fibers are the trajectories of a complete unit Killing vector field. In this paper, we give a characterization of proper biharmonic CMC surfaces in a Killing submersion. In the last part, we also classify the proper biharmonic Hopf cylinders in a Killing submersion.2000 Mathematics Subject Classification. 53C42, 58E20.1 is the curvature operator on (N, h). Equation (4) is a fourth order semi-linear elliptic system of differential equations. We also note that any harmonic map is an absolute minimum of the bienergy and, so, it is trivially biharmonic. Therefore, a general working plan is to study the existence of proper biharmonic maps, i.e. biharmonic maps which are not harmonic. We refer to [4,12] for existence results and general properties of biharmonic maps.In this paper, we study biharmonic surfaces in the total space of a Killing submersion. A Riemannian submersion π : E → M of a 3-dimensional Riemannian manifold E over a surface M will be called a Killing submersion if its fibers are the trajectories of a complete unit Killing vector field ξ. Killing submersions are determined by two functions on M, its Gaussian curvature G and the so called bundle curvature r (for more details, see [9,11] and Section 2) and, for this reason, we denote them by E(G, r). A remarkable family of Killing submersions are the homogeneous 3-spaces. In fact, with the exception of the hyperbolic 3-space H 3 (−1), simply-connected homogeneous Riemannian 3-manifolds with isometry group of dimension 4 or 6 can be represented by a 2-parameter family E(c, b), where c, b ∈ R. These E(c, b)-spaces are 3-manifolds admitting a global unit Killing vector field whose integral curves are the fibers of a certain Riemannian submersion over the simply-connected constant Gaussian curvature surface M(c) and, therefore, they determine Killing submersions π : E(c, b) → M(c) (for more details, see [6]). A local description of these examples can be given by using the so called Bianchi-Cartan-Vranceanu spaces (see Section 2). As it turns out, the E(c, b)-spaces are the only simply-connected homogeneous 3-manifolds admitting the structure of a Killing submersion (see [11]).