This paper proposes a new observer for estimating the state vector of a nonlinear system. This observer, which is robust with respect to outliers, assumes that the measurement errors, as well as the number of outliers that could occur within a given time window, are bounded. The principle of the approach is to use interval analysis to deal properly with the nonlinearities involved in the system (without any linearization or approximation) and to propagate through the time, in a forward and backward manner, the assumptions made about outliers. A test case related to the localization and control of an underwater robot is also proposed to illustrate the efficiency of the approach. (C) 2008 Elsevier Ltd. All rights reserved,