In this paper we design an interval observer for nonlinear uncertain continuous-time dynamical systems, in the unknown-but-bounded error framework. We show how to generate two coupled hybrid observers, which yield the best conservative upper and lower component-wise bounds for the state, provided bounds are available for initial conditions and uncertainties. The interval observer depends on a tuning gain, which is chosen to satisfy the practical stability of a given hybrid dynamical system. An example is studied to emphasize the performance of our method. Copyright © 2010 John Wiley & Sons, Ltd.