Timoci Nagusa scored a hat-trick as Montpellier moved to the summit of the Top 14 with a 38-17 win over Lyon after Bordeaux-Begles beat La Rochelle on Saturday.
Vern Cotter's men were given the opportunity to go top when La Rochelle slipped up and they grasped it with both hands at Altrad Stadium.
Montpellier had slumped to back-to-back defeats in the French top flight before beating Glasgow Warriors twice in the European Champions Cup, but dominated the second half to see off Lyon.
Nagusa was the star of the show, the Fiji winger coming off the bench to claim a second-half treble after Louis Picamoles scored two tries in the first half.
Joe Tomane added a sixth try late on for the new leaders, with five pointers from Liam Gill and Thibault Regard in vain for Lyon.
La Rochelle are three points behind Cotter's side following a 29-19 loss to Bordeaux at Matmut Atlantique.
After losing to Wasps in the Champions Cup last week, La Rochelle were found out again despite Bordeaux having Jean-Baptiste Dubie's red card in the second half for a high tackle on Vincent Rattez.
Geoffrey Cros and Peni Ravai were Bordeaux's try-scorers, while Matthieu Jalibert scored 19 points with the boot in La Rochelle's fourth Top 14 loss of the season.
Castres are up to third after a 28-6 win over Stade Francais and Agen beat fellow strugglers Brive 27-13.
Pau edged out Clermont Auvergne 22-21 and Toulon scored six tries in a 49-25 rout of bottom side Oyonnax.