1. A method of estimating the fuel/air ratio in each cylinder of a multicylinder internal-combustion engine comprising an exhaust circuit including at least pipes connecting the exhaust of the cylinders to a manifold and a fuel/air ratio detector downstream from said manifold, characterized in that it comprises the following stages: establishing a physical model (RTM) representing in real time the expulsion of the gases from the cylinders and their travel in said exhaust circuit to the detector, coupling said model with a non-linear state observer of Extended Kalman Filter type wherein the fuel/air ratio measurement provided by the detector is taken into account, deducing the fuel/air ratio value at the exhaust circuit inlet.