Biarritz missed the chance to go third in the Top 14 as they were beaten 26-19 at Castres.
The Atlantic coast club have won on the road four times already this season but Romain Teulet booted 16 points at the Stade Pierre-Antoine to deny them victory.
Steve Kefu scored an early try for the hosts which was quickly cancelled out by Takudzwa Ngwenya.
Laloaoa Milford then crossed eight minutes before the break to give Castres a 20-10 lead at the interval.
Teulet booted two second-half penalties and although Marcelo Bosch replied with three of his own for the visitors, Castres held on to move into fifth place.
League leaders Toulouse will hope to bounce back from their loss at Bourgoin last week - only their second of the season - when they travel to Bayonne on Saturday.
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Top 14 - Castres go fifth with victory
Top 14 - Castres go fifth with victory
