Champs Clermont falter again at Castres
Champs Clermont falter again at CastresSHARE
Meanwhile, Montpellier went back to the Top 14 summit by thrashing Brive 54-10.
Castres were in control from the moment South Africa hooker Jody Jenneker gave them a seventh-minute lead with the first try of the game.
The champions, who have lost all three away games this season, found themselves 0-14 down on the quarter hour after New Zealander flank Maama Vaipulu also crossed the whitewash.
Argentine flyhalf Benjamin Urdapilleta then kept the score ticking over with five penalties to finish with a personal haul of 19 points.
Tries from Australian back Peter Betham and replacement hooker Yohan Beheregaray, as well as 13 points from the boot of Scotland scrumhalf Greig Laidlaw, wasn't enough for Clermont, who remain ninth ahead of a trip to rejuvenated Toulouse next week.
They are already 11 points behind Montpellier, who claimed a fifth win from six games as giant Fijian winger Nemani Nadolo bagged a hat-trick of tries.
Nadolo's treble was part of an eight-try, bonus-point victory that saw Montpellier leapfrog Lyon and Toulouse into first place on 24 points.
Brive, however, remain winless from their opening six matches and in bottom spot on just one point.
Australian fullback Jessie Mogg, one of 11 foreigners in Vern Cotter's starting XV, opened the scoring with a soft, opportunistic try, following up on his deft through kick with no defenders to be seen.
Springbok and captain Bismarck du Plessis was bundled over from a driving maul, while Nadolo crashed in at the corner in quick succession.
With Brive struggling to contain Montpellier's powerful pack, Du Plessis was driven over for his second in identical fashion to the first, and Nadolo then got his second, intercepting a forced pass by Benjamin Lapeyre to sprint clear.
With South African scrumhalf Ruan Pienaar looking sharp off the conversion tee, Brive were down 3-33 at half-time and looking at damage limitation.
But the visitors showed more attacking intent and a sharper defence early in the second period, although they could do nothing when a Montpellier scrum threatened to roll, referee Thomas Charabas awarding a penalty try as they attempted to disrupt it.
Replacement flank Fulgence Ouedraogo then crossed after pressure saw Brive spill up the ball, All Black Aaron Cruden converting to extend Montpellier's lead.
Arnaud Mignardi sliced through for a well-taken consolation try six minutes from the end, but it was Nadolo who had the last word, making the most of an excellent Alexandre Dumoulin pass to wrap up his hat-trick with the Brive defence split.
First Toulouse, in thumping Agen 30-10, and then Lyon later in the day by edging Racing 92 20-17 in Paris, had gone top on Saturday.
South African wing Cheslin Kolbe and France centre Gael Fikou crossed the whitewash either side of a penalty try for Toulouse to briefly send them top of the league on Saturday, as Thomas Ramos kicked 11 points.
But tries from wings Alexis Palisson and Toby Arnold led Lyon to a surprise win over 2016 champions Racing.
Earlier on Saturday, La Rochelle had missed the chance to go top as Anthony Belleau again proved their downfall against Toulon.
Belleau kicked 14 points as Toulon held off a fierce La Rochelle fightback to win 26-20 just three months since the flyhalf's last-gasp drop goal sent his club into the Top 14 final at La Rochelle's expense.
Toulon 26 La Rochelle 20
Oyonnax 9 Bordeaux-Begles 39
Pau 23 Stade Francais 25
Toulouse 30 Agen 10
Racing92 17 Lyon 20
Castres 29 Clermont 23
Montpellier 54 Brive 10