We present in this paper a fasi method to delermine the posiiion and orieniaiion of a mobile robol. An external vision system covering the work area is used o localize the roboL A known paflern composed of coplanar poinis fixed on the robot is exiraced from a single image and used o locate Ihe robot in 3D. Assuming iha the robol's movemenis are locally planar, the exiracled poinis are projecled on ike paUern's plane, estimaied from the previous localization. An iterative ioop to match paflern points and to localize the paUern is then performed. From four idenlified points, a 3D localization is possible.