Résumé : The reactive component of a motion control architecture (called EM) for a car-like vehicle intended to move in dynamic and partially known environments is presented in this paper. Its purpose is to generate commands for the servo-systems of the vehicle so as to follow a given nominal trajectory while reacting in real-time to unexpected events. EM is designed as a fuzzy controller. A behaviour-based approach is used to set up the fuzzy rule base: the overall behaviour of the vehicle results from the combination of several basic behaviours (trajectory following, obstacle avoidance, etc.). This approach permits an easy and incremental construction of the fuzzy rule base and also to develop and test the basic behaviours separately. EM has been implemented and tested on a real computer-controlled car equipped with sensors of limited precision and reliability. Experimental results obtained with the prototype vehicle are presented. They demonstrate the capability of EM to actually control a real vehicle and to perform trajectory following and obstacle avoidance in real outdoor environments by using simple fuzzy behaviours relying upon low-resolution sensor data.