Five-time winners Biarritz were relegated from the French Top 14 on Saturday after their 48-13 loss to Castres and Perpignan's 27-10 victory over Brive.
The Basque outfit, promoted last term, had to win and hope Perpignan failed to pick up a point until the end of the campaign to maintain their status in the top-flight.
Fiji winger Filipo 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.
"The squad found it hard against a better team. We shouldn't look for excuses," Biarritz head coach Shaun Sowerby told reporters.
"There's no carrot (just a stick), some will go elsewhere, some will retire, it's hard to keep everyone focused," he added.
In Perpignan, Samoa back-rower Piula Faasalele's double came either side of the break and lifted the 13th-placed Catalans two points behind Brive in the last place that guarantees safety, with two rounds of the regular season remaining.
If the 2009 champions fail to jump above Brive then they will face the runners-up from the second-tier ProD2 in a one-off decider.
"We're still alive. It's not all down to us," Perpignan's France full-back Melvyn Jaminet said.
"We're going to do everything to avoid the relegation play-off, but if we have to play it we'll play it as we need to," he added.
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 Mohamed 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 beat La Rochelle 23-16 in a rerun of last season's Top 14 and European Champions Cup finals to move into the sixth and final play-off spot.
Nyakane's fellow Springbok Cheslin Kolbe was jeered when his face appeared on the big screen with the full-back watching his former side, the double winners, from the stands after leaving for Toulon in September.
On Sunday, second-placed Bordeaux-Begles can take over at the top of the league table if they beat Kolbe's outfit, 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.