Paris - La Rochelle snatched pole position in the French Top 14 on Saturday with a bruising 22-8 win over an undisciplined Castres who gave up 20 penalties and finished the night a man short.
Victory allowed La Rochelle to move four points clear of Clermont who face Toulon on Sunday.
After Brock James and Benjamin Urdapilleta had exchanged early penalties, it was Castres who scored the first try on a bitterly cold night.
South Africa hooker Jody Jenneker barrelled over from short range for an 8-3 lead but Urdapilleta missed the extras.
Zack Holmes cut the deficit for La Rochelle with a penalty as his team trailed 8-6 at the break.
Castres finished the opening period with Uruguayan skipper Rodrigo Capo Ortega in the sin bin for a dangerous tackle.
No sooner had he returned early in the second half than scrumhalf Rory Kockott was also yellow-carded for a similar offence.
Holmes kicked La Rochelle ahead and they never looked back with All Blacks World Cup winning flanker Victor Vito scoring a try after being set-up by veteran Springboks scrumhalf Ricky Januarie.
Holmes slotted over the conversion for 16-8 before adding two further penalties for a comfortable win.
Castres played the last 10 minutes a man short once again when replacement prop Yohan Montes was red-carded for appearing to aim a head butt at Mohammed Boughanmi who was yellow carded for his role in the affray.
Montes had only been on the field for five minutes before he was sent packing.
Earlier Saturday, Brive pushed Grenoble closer to relegation with a 23-22 win with the Top 14's top scorer Gaetan Germain kicking 18 of his side's points.
The match between Pau and Lyon, meanwhile, was postponed due to an icy pitch.
On Friday, Montpellier scored three tries through Fijian winger Nemani Nadolo, centre Vincent Martin and former Australian rugby league star Joe Tomane in a 31-26 win against faltering Bordeaux.
The win allowed Jake White's third-placed team to bury the shock 32-27 loss at Pau last week while Bordeaux have now lost for games in succession.
The only consolation for Bordeaux was Simon Hickey's last-gasp penalty allowing them to claim a defensive bonus point.