Tanguy Molcard's last-minute try earned Biarritz victory at Worcester.
Dean Hammond's powerful run saw Worcester take a 5-0 lead with Alex Grove scoring their second try soon after which Chris Pennell converted.
But Taku Ngwenya scored for Biarritz just before the break with Julien Peyrelongue converting.
Yann Lesgourgues' try and Peyrelongue's conversion put Biarritz ahead but Pennell kicked a penalty before Molcard crossed late on to win it.
Worcester at least earned a losing bonus point but their progress in the Amlin Challenge Cup looks unlikely.
The result sees Biarritz move to within one point of Pool One leaders Sale, who lost at Oyonnax, with Worcester bottom.
Worcester: Pennell, Stephenson, Grove, Matavesi, Hammond, Warwick, Su'a, Fainga'anuku, Shervington, O'Donnell, Percival, Galarza, De Carpenier, Betty, Thomas.
Replacements: Stelling for Matavesi (57), Hodgson for Su'a (50), Williams for Fainga'anuku (69), Seymour for Shervington (69), Porter for Percival (69).
Not Used: Andress, Mieres, Jones.
Sin Bin: Fainga'anuku (35).
Biarritz: Couet-Lannes, Ngwenya, Sella, Gimenez, Brew, Peyrelongue, Roussarie, Barcella, Dabti, Broster, Lockley, Marie, Furno, Molcard, Fono.
Replacements: E. Lund for Sella (41), Baby for Gimenez (62), Lesgourgues for Roussarie (58), van Staden for Broster (50), Guyot for Lockley (65), Waenga for Marie (41), Synagehel for Furno (48).
Not Used: Whittaker.
Sin Bin: Barcella (48).