Castres' reign as Top 14 champions is over after they were beaten 25-16 by Toulon as La Rochelle and Montpellier sealed play-off places on a dramatic final day of the regular season.
Christophe Urios' side had their fate in their own hands, but the holders were left shell-shocked at Stade Pierre-Fabre on Saturday.
Wing Daniel Ikpefan scored a try in each half and Anthony Belleau also crossed five minutes before the break after a five-pointer and a penalty from Benjamin Urdapilleta had eased Castres' nerves.
Jody Jenneker touched down three minutes from time but the damage had already been done, Belleau finishing with 15 points as Castres suffered a defeat that ended their campaign.
La Rochelle capitalised by putting Bordeaux-Begles to the sword, Vincent Rattez scoring a hat-trick in an 81-12 rout.
Kini Murimurivalu and Wiaan Liebenberg helped themselves to doubles as La Rochelle racked up 12 tries to finish fifth.
Last year's runners-up Montpellier prolonged their season with a tense 28-27 win at Clermont Auvergne, Youri Delhommel grabbing a brace of tries and Nemani Nadolo scoring to seal sixth place.
Racing 92 clinched fourth by thumping Agen 35-3, while Toulouse took their tally to an astonishing 98 points for the regular campaign, thrashing relegated Perpignan 47-7.
There were also victories for Lyon and Stade Francais over Grenoble and Pau respectively.