Abstract. We describe a new class of Lie bialgebroids associated with Poisson-Nijenhuis structures.Résumé. Nousétudions une nouvelle classe de bigébroïdes de Lie, associés aux structures de Poisson-Nijenhuis.Introduction. Nijenhuis operators have been introduced in the theory of integrable systems in the work of Magri, Gelfand and Dorfman (see the book [4]), and, under the name of hereditary operators, in that of Fuchssteiner and Fokas. Poisson-Nijenhuis structures were defined by Magri and Morosi in 1984 [15] in their study of completely integrable systems. There is a compatibility condition between the Poisson structure and the Nijenhuis structure that is expressed by the vanishing of a rather complicated tensorial expression. In this letter, we shall prove that this condition can be expressed in a very simple way, using the notion of a Lie bialgebroid [14] [7] [12]. A Lie bialgebroid is a pair of vector bundles in duality, each of which is a Lie algebroid, such that the differential defined by one of them on the exterior algebra of its dual is a derivation of the Schouten bracket. Here we show that a Poisson structure and a Nijenhuis structure constitute a Poisson-Nijenhuis structure if and only if the following condition is satisfied: the cotangent and tangent bundles are a Lie bialgebroid when equipped respectively with the bracket of 1-forms defined by the Poisson structure, and with the deformed bracket of vector fields defined by the Nijenhuis structure.Let me add three "historical" remarks. This result was first conjectured by Magri during a conversation that we held at the time of the Semestre Symplectique at the Centre Emile Borel. Secondly, the Lie bracket of differential 1-forms on a Poisson manifold, defining the Lie-algebroid structure of its cotangent bundle, was defined by Fuchssteiner in an article of 1982 [6] which is not often cited, though it is certainly one of the first papers to mention this important definition. Thirdly, as A. Weinstein has shown [18] [19], Sophus Lie's book [11] contains a comprehensive theory of Poisson manifolds under the name of function groups, including, among many results, a proof of the contravariant form of the Jacobi identity, a proof of the duality between Lie algebra structures and linear Poisson structures on vector spaces, the notions of distinguished functions (Casimir functions) and polar groups (dual pairs), and the existence of canonical coordinates. Moreover, Carathéodory, in his book [2], proves explicitly the tensorial character of the Poisson bivector and gives a rather complete account of this theory, based on a short article by Lie [10] that appeared even earlier than the famous "Theorie der Transformationsgruppen", Part II, of 1890.