Ben Cohen and Philippe Bibabe will meet again in the last eight
Northampton (3) 8
Biarritz (3) 17
Try: Masi, Bobo
Cons: Yachvili (2)
Northampton will have to travel to Biarritz in the quarter-finals after the French side eased to victory in their final Heineken Cup group game.
Northampton had the best of the first half but all they had to show for it was Carlos Spencer's penalty, levelled by Dmitri Yachvilli's drop goal.
After the restart Andrea Masi burst throught to score a try and set up Sireli Bobo for a second.
Sonae Tonga'uiha's late try restored some pride for Saints.
Biarritz rarely got out of first gear in the first half, with the sin-binning of Bobo and David Couzinet summing up their negative approach.
But Spencer's misjudged restart kick set up a period of Biarritz pressure, culminating in Masi's touchdown.
Yachvili and Masi then did the approach work before Bobo flew through the Saints defence as they demonstrated they would control the game at will.
Replacement Tonga'uiha's late pushover try ensured Saints, whose qualification was guaranteed on Saturday when other results went theit way, ended on the front foot but a comeback was never likely.
Northampton coach Paul Grayson:
"It was always going to be hard. If we could have crossed early that'd have put some doubt in their mind.
"Biarritz don't do what they do by accident and they win games by conceding very few points and taking their chances.
"We've got a lot of our plate before the quarter final."
Northampton: Kydd; Sean Lamont, Cohen, Quinlan, Diggin; Spencer, Robinson; Budgen, Thompson (capt), Barnard, Gerard, Short, Tupai, Fox, Browne.
Replacements: Hartley, Tonga'uiha, Lord, Easter, Howard, Laird, Wyles.
Biarritz: Dambielle; Bidabe, Gaitan, Masi, Bobo; Peyrelongue, Yachvili; Balan, August (capt), Avril, Thion, Couzinet, Betsen, Harinordoquy, Lievremont.
Replacements: Noirot, Lecouls, Dellappe, Dridi, Dupuy, Marlu, Padro.