Connacht were unable to repeat their heroics of a week ago as Biarritz won 17-0 in horrendously wet conditions at the Parc des Sports Aguilera.
Man of the match Dimitri Yachvili kicked an early penalty and converted Iain Balshaw's 31st-minute try to give Biarritz the upper hand and a 10-0 interval lead in the Heineken Cup clash.
There then followed a stop-start second half during which Tiernan O'Halloran and Eoin McKeon caught the eye for the hard-working visitors.
Biarritz were meandering to victory and put the finishing touches to a good night when Damien Traille latched onto his own kick, taking advantage of a fortuitous bounce to put replacement centre Seremaia Burotu over in the final minute.
Yachvili converted superbly with the last kick of the match, the result of which sees Biarritz leapfrog over Connacht into second place with Harlequins looking like runaway winners of Pool Three.
The saturated pitch contained pools of water in certain areas but the ground staff cleared enough of it for the fixture to go ahead as scheduled.
Connacht were expecting a backlash from the French giants, who sacked coaches Jack Isaac and Serge Milhas in the aftermath of their shock 22-14 defeat in Galway.
Laurent Rodriguez and Mathieu Rourre were installed as an interim coaching tandem and they would have been pleased with Biarritz's start.
Their forwards rumbled into the Connacht 22, winning two early penalties, with Yachvili sticking a third-minute kick through the uprights.
As heavy rain continued, Connacht dug their heels in and defended manfully with young full-back Robbie Henshaw secure under the high ball and the mud-covered pack competing in stout fashion.
Biarritz had the first sight of the try line in the 12th minute, Takudzwa Ngwenya sliding towards the right corner after set-piece ball was swung wide. However, the United States international failed to get the ball down with the Connacht cover doing just enough.
Dan Parks, so influential with a 17-point haul seven days ago, went the aerial route in a bid to give the visitors some momentum. Biarritz were territorially dominant though and exerted more control via clever kicking from Traille, Balshaw and Yachvili.
They made a breakthrough on the half-hour mark as they went left from a midfield scrum, with Traille's half break making the initial incision and Jean-Pascal Barraque passing precisely for the supporting Balshaw to splash over in the corner.
Yachvili supplied an excellent conversion from wide out.
Late on in the first half, Eoin McKeon took out the France scrum-half off the ball, but Yachvili slid his resulting penalty effort to the left and wide.
A sin-binning for Biarritz hooker Benoit August, who got involved in some afters with Johnny O'Connor, presented Connacht with an opportunity to launch themselves forward.
However, Parks sent a subsequent penalty from a difficult position wide and some handling errors robbed Connacht of precious momentum in an evenly fought third quarter.
An injury to Connacht centre Danie Poolman, who got an accidental knee to the head at a ruck, broke up play for a number of minutes as he was taken off on a stretcher.
Aled Brew ruined a good scoring chance for the home side with a sloppy pass, but they remained on the front foot despite losing replacement Thibault Dubarry to the sin bin.
Biarritz pressed from a Fabien Barcella surge into the Westerners' 22 and a five-metre scrum. Connacht held out until Traille opened the way for Burotu to power past Paul O'Donohoe's last-ditch tackle in the left corner.