Samoa back-row Piula Faasalele scored twice as Perpignan beat Brive 27-10 to relegate bottom side Biarritz from the French Top 14.
The Basque outfit, five time French champions, had to beat Castres at home and hope Perpignan failed to pick up a point until the end of the campaign to maintain their status in the top-flight but they lost 48-13.
Fiji winger Filipi Nakosi was one of seven try scorers for Castres, who go up to third, opening his league account for the season after touching down 12 times last term.
Biarritz replied with a Darly Domvo try and Brett Herron's attempts at goal but those were not enough to keep a squad, which includes ex-All Blacks centre Francis Saili and former Australia backs Henry Speight and Tevita Kuridrani, in the Top 14.
In Perpignan, veteran Faasalele's contributions came either side of the break and lifted the 13th-placed Catalans two points away from Brive in the last place that guarantees safety, with two rounds of the regular season remaining.
Elsewhere, South Africa's Rugby World Cup winner Trevor Nyakane claimed his first Racing 92 try in their 42-21 victory at Pau to move the Parisians up to fourth and a step closer to a home play-off.
- Montpellier humbled -
Earlier, Lyon thrashed leaders Montpellier 43-20 to pull themselves into contention for the end of season knock-outs.
Montpellier finished the game with 14 men after France prop Mohammed Haouas was sent off just before half-time and lost a third straight game in all competitions for the first time this season.
"This season we have had better days," said Montpellier coach Philippe Saint-Andre.
"It's the risks of high-level matches. Lyon were very pragmatic and very powerful in contact," he added.
Later on Saturday, Toulouse host La Rochelle in a re-run of last season's Top 14 and European Champions Cup finals in front of a sold-out 30,000-capacity Le Stadium.
On Sunday, second-placed Bordeaux-Begles can take over at the top of the league table if they beat Toulon and welcome France fly-half Matthieu Jalibert back after more than three months out with a thigh injury.
Next weekend, Montpellier head to La Rochelle in the Champions Cup last eight and could be without Jalibert's Test team-mate lock Paul Willemse who went off after 25 minutes with a knee injury.