The Magners League leaders led 20-11 early in the second half following scintillating tries from Thom Evans and Chris Cusiter, but Biarritz rallied with 30 unanswered points, clinching the crucial bonus point 50 seconds from the final whistle to guarantee home advantage in the last eight. Eduard Coetzee and Takudzwa Ngwenya crossed before Ilikena Bolakoro followed a penalty try to wrap up maximum points and inflict a fourth defeat on Glasgow.
"We were pleased with the way the first half went and we showed we can score tries at any level against the best sides, despite the wet conditions," said Glasgow coach Sean Lineen. "But you saw the strength of Biarritz with the guys they brought off the bench and we struggled to match them up front. It's a disappointing way to end the campaign but a lot of the young guys will have learned from this experience."
Scotland skipper Chris Cusiter was at the centre of an injury scare after just 60 seconds as he went down clutching his already troublesome calf. However, the scrum-half was able to resume after a lengthy spell of treatment.
The action was scrappy on the greasy surface and it was the hosts who broke the deadlock with a 40-metre penalty by Valentin Courrent after Richie Gray was caught offside at a ruck, but the Warriors replied in positive style to create a magnificent try for winger Thom Evans. Cusiter inflicted the damage with a chip down the narrow side and the pace of Evans was too hot for Biarritz and he scooped up the ball to skid over in the corner.
Dan Parks provided a further morale boost by banging over the conversion from the widest possible angle.
Clearly stung by the Glasgow score, the Frenchmen began to dominate the exchanges in terms of possession and territory. Eventually the Warriors shook off the shackles to earn a penalty for failure to vacate the tackle scene. Parks then made light of the worsening weather to hit the target from 35 metres.
Second-row forward Richie Gray's charge led to a second Parks penalty, and a neat interchange of passes between Max Evans, John Beattie and Cusiter ended in a third as the visitors stunned the partisan home crowd by leading 13-3. That sparked a change in tempo and tactics from Biarritz who reduced the deficit to 13-11 by half-time. Valentin Courrent kicked a 28th-minute penalty but was off target five minutes later, and he missed again after prop Coetzee was shoved over the line by the rest of the Biarritz pack three minutes before the break.
The second half got off to a tremendous start for Glasgow, despite Parks missing for the first time, with a contender for try of the season in the Heineken Cup so far.
Full-back Bernardo Stortoni launched an audacious attack with a quickly taken penalty just ten metres from the Glasgow try-line, and it ended with Cusiter sliding between the Biarritz posts.
Stortoni burst from defence and exchanged passes with Max Evans before handing Cusiter a 25-metre dash to score. Parks' conversion moved Glasgow 20-11 ahead after 48 minutes but that was to be the last time they had any grip on the tie.
Biarritz turned their fortunes around in the scrum, with replacement Kiwi prop Campbell Johnstone having an impact, and with more firepower following off the bench that led to 30 unanswered points.
Courrent drew Biarritz level with three penalties before wing Ngwenya made light work of Hefin O'Hare's flapping attempt at a tackle to race clear for a try that put the hosts ahead for the first time since the tenth minute. The home side's advantage in the scrum forced referee Dave Pearson to award a penalty try after 69 minutes, and they clinched the vital bonus point when Bolakoro raced on to Imanol Harinordoquy's break to score in the last minute.
Scorers: Biarritz: Tries: Coetzee, Ngwenya, pen try, Lesgourgues. Cons: Courrent 3. Pens: Courrent 5. Glasgow: Tries: T Evans, Cusiter. Cons: Parks 2. Pen: Parks 2
Biarritz: P Couet-Lannes; T Ngwenya, A Mignardi, L Tranier, I Bolakoro; V Courrent, Y Lesgourgues; E Coetzee, B August, M Moala, P Taele, J Thion (capt), M Lund, T Hall, I Harinordoquoy. Subs: Balshaw for Couet-Lannes (66), Hunt for Lesgourgues (54), Hughes for Coetzee (71), Terrain for August (71), Johnstone for Moala (45), Carizza for Taele-Pvihi (67), Alaxndre for Hall (70).
Glasgow: B Stortoni; R Dewey, M Evans, G Morrison, T Evans; D Parks, C Cusiter; J Welsh, D Hall, M Low, R Gray, A Kellock (capt), K Brown, J Barclay, J Beattie. Subs: O'Hare for Dewey (56), Jackson for Parks (64), McMillan for Cusiter (65), Tkachuk for Welsh (61), Thomson for Hall (59), Turner for Low (41), Eddie for Beattie (59).