An output feedback regulator that is based on an extended Luenberger observer design is considered. It concerns the class of SISO (single-input, single-output) exponentially stabilizable and locally observable nonlinear systems. The state estimate used to implement a suitable regulation law is proved to lead to exponential stability around the origin: a quadratic Lyapunov function is obtained and a convergence domain is characterized. The stability analysis only deals with a full-order observer. However, the Lyapunov analysis is readily extended to the reduced-order case when the output is a component of the state. >