In this paper, we present the design of an observer for a special class of nonlinear systems satisfying some regularity assumptions in both the single-output and multioutput case. The controlled part of the considered systems have a triangular structure. A further extension of the design strategy is made to a specific class of multioutput systems which assume stronger nonlinear dependence between the various subsystems. The design is illustrated by a simulation experiment dealing with rotor flux estimation in an induction motor.