Castres bounced back from last week's loss to beat Montpellier 29-23 in the Champions Cup, keeping alive their slim hopes of qualification.
Akapusi Qera got the opening try for Montpellier, but Horacio Agulla's score put Castres in 16-10 in front.
Montpellier were far better in the second-half though, Jesse Mogg and Nemani Nadolo going over soon after the break.
They were dealt a huge blow though when Paul Willemse was sent off a minute after coming off the bench, with Alexandre Bias going over for Castres.
Having rung the changes from the return fixture last week, Castres were hoping to return the favour after the Montpellier defeat.
With both sides desperate for victory to keep up with Leinster, the hosts started the stronger and almost grabbed the opening try within three minutes. Rory Kockott and Agulla combined well, but they weren't able to drag the ball over the line.
Benjamin Urdapilleta did however kick his team three points ahead though.
Neither side could take full control of the game, and a Demetri Catrakilis only momentarily levelled for the visitors, with Urdapilleta restoring the three point lead immediately after.
Montpellier finally made a move though, and they claimed the opening try on 26 minutes. Having spotted a gap in the defence, Catrakilis fed the ball to Qera who went over on the right. The conversion took Montpellier to double figures.
But Castres had the reliable boot of their number 10 to thank for staying within touch, as he kicked another three to reduce the gap to a point.
And they went into the lead ahead thanks to Agulla. Having set themselves up in the 22, Urdapilleta's kick sailed into the arms of Agulla who went through on the right wing. The conversion followed, and Urdapilleta had played a huge part in taking a 16-10 lead into the break.
But that didn't last for long though, as Montpellier stormed out of the blocks after the break. Nadolo battered his way through two challenges, using that space to offload to Mogg who went in unchallenged, though the conversion miss kept Castres in front.
The visitors' 10 made no mistake with his next kick, putting Montpellier two points in front. And Nadolo stretched that lead after intercepting a pass on halfway and putting the miles in to go over in the corner.
The extras didn't follow, with Castres' Urdapilleta reducing the lead back down to four points.
The game changed from then on, because Montpellier found themselves a man down for the last 20 minutes. Willemse – who had been on the pitch for little more than a minute – took Kockott out in the ruck, a shoulder to head challenge, leaving Wayne Barnes with little option but to get the red card out.
14 became 13 as Jannie du Plessis was shown a yellow, with Bias going over barely a minute after. A line-out saw the replacement find the space to go over, Urdapilleta making it 26-23.
Kockott made it a six-point lead with a wonderful kick from distance, and with only four minutes left on the clock, the man short Montpellier were given too big a task to turn it around. Though they had possession and ground late on, a knock-on proved the final straw as Castres secured the win.
Tries: Agulla, Bias
Cons: Urdapilleta 2
Pens: Urdapilleta 4, Kockott
Tries: Qera, Mogg, Nadolo
Pens: Catrakilis 2
Yellow Card: Du Plessis
Red Card: Willemse
Castres: 15 Geoffrey Palis, 14 Julien Caminati, 13 Afusipa Taumoepeau, 12 Florian Vialelle, 11 Horacio Agulla, 10 Benjamin Urdapilleta, 9 Rory Kockott, 8 Anthony Jelonch, 7 Steve Mafi, 6 Yannick Caballero, 5 Victor Moreaux, 4 Loïc Jacquet, 3 Damien Tussac, 2 Marc-Antoine Rallier, 1 Mihaita Lazar
Replacements: 16 Brice Mach, 17 Tudor Stroe, 18 Yohan Montes, 19 Thibault Lassale, 20 Alexandre Bias, 21 Antoine Dupont, 22 Julien Dumora, 23 Pierre Berard
Montpellier: 15 Jesse Mogg, 14 Timoci Nagusa, 13 Joe Tomane, 12 Alexandre Dumoulin, 11 Nemani Nadolo, 10 Demetri Catrakilis, 9 Nic White, 8 Akapusi Qera (c), 7 Kélian Galletier, 6 Fulgence Ouedraogo, 5 Nicholaas Van Rensburg, 4 Konstantine Mikautadze, 3 Jannie du Plessis, 2 Shalva Mamukashvili, 1 Mikheil Nariashvili
Replacements: 16 Romain Ruffenach, 17 Yvan Watremez, 18 Antoine Guillamon, 19 Paul Willemse, 20 Antoine Battut, 21 Cameron Wright, 22 Henry Immelman, 23 Yvan Reilhac
Referee: Wayne Barnes (England)
Assistant Referees: Peter Allan (England), Paul Dix (England)
TMO: Rowan Kitt (England)