La Rochelle's stunning form saw them pull clear of Clermont Auvergne at the Top 14 summit on Sunday, winning 27-21 at Toulouse.
Les Corsaires have stormed into contention for a maiden top-flight crown with a run of six consecutive victories stretching back to December 23. In all competitions, the winning sequence stands at nine matches.
Having headed into the weekend level on points with Clermont, defeat for the 2010 champions at Castres on Saturday opened the door for La Rochelle to establish a four-point cushion at the top.
And the visitors edged a tight affair at the home of the 19-time French champions, with four Zack Holmes penalties laying the foundation as La Rochelle ended the first half 12-6 in front.
Joe Tekori closed the gap with a try 12 minutes into the second period, but Jone Qovu and Pierre Aguillon touched down for La Rochelle to open up a 27-14 lead.
Julien Marchand crossed the whitewash seven minutes from time to set up a tense finish, but the visitors held on for their fifth away win of the season.
Montpellier missed a chance to go third as they were beaten 16-3 at lowly Lyon - Willie du Plessis' penalty just short of the hour proving the visitors' only consolation and the last points-scoring action of the match.
In the day's remaining fixture, Stade Francais ended a three-match losing run by easing to a 32-9 home win against Bordeaux Begles.