A seismic swarm occurred in the south-east of France in December 2000, about 15 km north of the densely populated cities of the French-Riviera. More than 300 events were located by a dense temporary seismic network that had been installed in the epicentral region one month before. We evidence an alignment of the seismicity that we interpret as an unknown active fault. On the basis of regional tectonics, we propose that this fault results from basement faulting cutting through the detached overlying sedimentary cover.