In this paper, the authors propose an observer for continuous-time nonlinear systems. The observer gain is computed by a Riccati differential equation similar to the extended Kalman filter. They prove that under certain conditions the proposed observer is an exponential observer by choosing an appropriate Lyapunov function. Furthermore, the authors explore some important relations of the proposed observer to robust control theory and H-infinity-filtering, To examine the practical usefulness of the proposed observer they applied it to an induction motor for the estimation of the rotor Bur and the angular velocity.