Résumé
The purpose of this paper is to study the equidistribution properties of the horocycles of a negatively curved surface with infinite volume. Recall that on the unit tangent bundle T(1) S of a compact hyperbolic surface S, the horocyclic flow (h(s)) is uniquely ergodic (Furstenberg [13]). In particular, all orbits are equidistributed with respect to the unique invariant measure: the Liouville measure. If S is not compact, but has finite volume, the periodic orbits induce other ergodic invariant probability measures (Dani [9]), but the result is almost the same (Dani and Smillie [10]): all nonperiodic orbits are equidistributed with respect to the Liouville measure.