The present invention relates to a method of estimating the fuel/air ratio in each cylinder of an injection internal-combustion engine comprising an exhaust circuit on which a detector measures the fuel/air ratio of the exhaust gas. An estimator based on an adaptive nonlinear filter is coupled with a physical model representing the expulsion of the gases from the cylinders and their travel in the exhaust circuit to the detector. The estimator is also coupled with an estimation of the fuel/air ratio measured from at least one variable of said model such as the total mass of exhaust gas and the mass of fresh air. The method has application to engine controls.