Toulouse full-back Matthis Lebel might have scored a contender for try of the season, but it was not enough as Castres held their nerve to claim a 26-24 Top 14 derby scalp on Saturday.
Lebel and Zack Holmes were the only two on Toulouse's teamsheet to have played the Top 14 leaders' last two European Champions Cup matches.
Of the 23 involved in victories over Munster (40-33) and Clermont (21-12), 13 didn't even make the trip for the derby in southwest France, meaning the likes of the France half-back pairing of Antoine Dupont and Romain Ntamack and South Africa's Rugby World Cup winner Cheslin Kolbe were missing.
It was a fifth victory in six matches for the hosts, who can now maintain hopes of making the top six play-off spots.
Three Benjamin Urdapilleta penalties for Castres were cancelled out by one from Holmes and the opening try of the encounter from Dimitri Delibes.
But it was Castres who went into half-time in the lead after Fijian centre Vilimoni Botitu crossed in the 36th minute.
Castres turned the screw with further tries from Thomas Combezou and Canada flanker Tyler Adron for a 26-10 lead before Toulouse sprung back into action in the last quarter.
First Thibault Flament went over, replacement scrum-half Baptiste Germain converting.
And then Lebel was on hand to finish off a sweeping length-of-the-field move sparked by Maxime Marty and which saw a sweet drop-off from Germain to Romain Riguet and eventually the ball passed to the full-back who stepped back inside two defenders for his 13th try of the season.
But even with Germain's conversion, Toulouse were left two points shy, much to Castres' derby joy.
La Rochelle play Lyon in Saturday's late game on a weekend which has seen Montpellier v Toulon, Agen v Bordeaux-Begles, Clermont v Brive, and Racing 92 v Stade Francais all postponed over Covid-19 fears.
Despite their game being called off, winless Agen were relegated from the Top 14 on Friday after second-bottom Pau defeated fellow strugglers Bayonne 43-33.
With just six games left of the regular season, Agen, who were eight-time national champions between 1930 and 1988 but have lost all 20 league games this season, cannot make up the gap.