The reigning Aviva Premiership champions ended Biarritz's four-year unbeaten home run in the competition to wrap up a dominant display in the pool stages.
Tom Guest grabbed the game's only try while the tactical and goal-kicking of Nick Evans, who scored 11 points, proved decisive.
Harlequins are guaranteed a home tie in the quarter-finals and only Toulon and Clermont-Auvergne can now deny them top seeding for the knockout stage.
Defeat ended Biarritz's slim hopes of progressing, though the 2006 and 2010 finalists could still qualify for the Amlin Challenge Cup as one of three group runners-up.
Connacht completed a hard earned double over Zebre as they wrapped up the Heineken Cup pool stages with a 25-20 victory.
The first half at the Sportsground passed with few incidents of note, Connacht gaining the upper hand thanks to three penalties and a drop goal from Dan Parks.
Eric Elwood's men led 12-6 at half-time, with Daniel Halangahu landing a penalty double for Zebre.
A penalty try should have inspired further tries from the home side, but Halangahu's boot closed the gap for the Italians and winger Ruggero Trevisan's well executed try set up a tense conclusion at 19-17.
Halangahu's fifth penalty goal had Zebre 11 minutes away from a famous first win, however Parks split the posts on two more occasions for a handsome 20-point haul.