Yachvili converted just one of Biarritz's four tries
Biarritz: (15) 22
Tries: Bobo (2), Dridi, August
Northampton: (3) 10
Sereli Bobo scored two tries as Heineken Cup favourites Biarritz held off a brave Northampton side.
Last season's runners-up scored four tries in the first 45 minutes but then allowed Saints back into the game.
Johnny Howard was the only points scorer for Northampton, pouncing on Ian Vass' neat chip for a 73rd-minute try.
Dimitri Yachvili's kicking for Biarritz was wayward all afternoon and Saints almost grabbed a late bonus point but were left to rue poor finishing.
The Saints' pack dominated the early lineouts, but had no answer to the size and pace of the French club's backline out wide.
Saints winger Sean Lamont showed brief glimpses of the same damaging running which Bobo created havoc with, but the Scotland international was woefully under-used.
Bobo's first try came after some terrific work at the breakdown from the Biarritz back row.
Howard, on the wing in the place of shoulder-injury victim Ben Cohen, gave the visitors their first points with a penalty goal after 20 minutes.
But the Saints had no answer to a wave of attacks and backrower Mohammed Dridi doubled his side's score with a try in the corner.
Saints wasted a couple of scoring opportunities and were made to pay as Biarritz powered back upfield and the impressive Bobo muscled his way over for his second try to leave the home side 15-3 up at half-time.
Biarritz secured a bonus point for four tries in the opening exchanges of the second spell, with hooker Benoit August being shoved over the line.
The try also brought Yachvili's first goal of the afternoon and with the bonus point won, the French side sat back.
Replacement Vass finally went over for Northampton and Howard's superb sideline conversion made it a 12-point match and gave the visitors a sniff of a losing bonus point but time ran out for Paul Grayson's men.
Northampton coach Paul Grayson:
"We simply made too many mistakes in the first half.
"We made all the mistakes we said we wouldn't make. In the second half, we said we would put the squeeze on them, and we did.
"We just lacked a bit of composure and firepower behind the scrum."
Biarritz: Brusque, Gobelet, Cabannes, Traille, Bobo, Peyrelongue, Yachvili, Balan, August, Avril, Thion, Couzinet, Betsen, Dridi, Noirot.
Replacements: Lealamanua, Wessels, Dellape, Padro, Dupuy, Lison, Dambielle.
Northampton: Laird, Lamont, Wyles, Quinlan, J. Howard, Myring, Robinson, Smith, Thompson, Budgen, D. Browne, Gerard, Tupai, Fox, D. Browne.
Replacements: Hartley, Emms, Lord, Easter, Vass, Mallon, Diggin.
Referee: Malcolm Changleng (Scotland)