Montpellier took advantage of an unexpected slip-up by La Rochelle at bottom club Oyonnax to go first in the Top 14 with a 28-16 win over Stade Francais on Saturday.
La Rochelle were held to a thrilling 38-38 draw by Oyonnax, as an 87th-minute Ben Botica penalty sealed a share of the spoils.
The strugglers led 18-17 at the break and Axel Muller's 44th-minute converted try stretched the advantage to eight points.
But La Rochelle scored three unanswered tries to seemingly put the contest beyond doubt, only for a late flurry to clinch two points for Oyonnax as Maxime Veau crossed before the boot of Botica denied the visitors in a shock outcome.
Montpellier, beaten by Brive last time out, returned to winning ways and the summit of the table as Aaron Cruden, Louis Picamoles, Mohammed Haouas and Nemani Nadolo scored tries in a bonus-point win over Stade Francais.
Third-placed Castres could have leapfrogged La Rochelle into second with a win at second-bottom Agen, but instead they slumped to a stunning 30-3 defeat.
They trailed 6-3 at the break but Agen stretched that lead to 20-3 by the time Maama Vaipulu was sent off on a miserable day for Castres.
Third-bottom Brive followed up their heroics against Montpellier by overcoming fifth-placed Toulon, who were undone by a last-minute try from their former back Benjamin Lapeyre in a 13-12 defeat.
Elsewhere, Bordeaux Begles beat Lyon 19-10 to move level on points with both them and sixth-placed Toulouse in the fight for a play-off spot.