Llanelli, beaten on home soil by Perpignan last season, departed at the same quarter-final stage and they could have few complaints.
First time semi-finalists Biarritz set up a potential showdown with reigning European champions Toulouse later this month, leaving Llanelli to lick their wounds.
Llanelli, consistently the great Welsh hope in Europe, trailed 15-3 midway through the second period after Biarritz scored tries from lock David Couzinet and full-back Nicolas Brusque.
And although fly-half Stephen Jones sparked a frenzied finale by converting his own try following Llanelli's one prolonged spell of pressure, it was too little, too late.
Brusque added his second touchdown on 79 minutes and substitute wing Philippe Bernat-Salles scored in injury time to finish Llanelli off, while scrum-half Dimitri Yachvili booted two conversions and fly-half Julien Peyrelongue landed a drop goal.
The capacity 10,800 crowd had painfully seen it all before, and Scarlets coach Gareth Jenkins still has not cracked the Heineken Cup code in terms of an appearance in European rugby's showpiece final.
Llanelli, bidding to reach their third Heineken Cup semi-final in five seasons, fielded a full-strength side.
Scarlets boss Jenkins made one change from the team that secured a last-eight spot through victory at Northampton two months ago, replacing lock Adam Jones with experienced Welsh international Chris Wyatt.
Biarritz included five players who featured during France's Grand Slam-winning RBS 6 Nations campaign - Brusque, Betsen, Yachvili, Peyrelongue and number eight Thomas Lievremont.
Both sides appeared riddled by nerves during an error-strewn opening, with poor handling and wayward kicking dominating the first 10 minutes.
Jones' first kick at goal - a 48-metre effort - hit the post below the crossbar, and Biarritz responded by trying to free speedster Jimmy Marlu, but they also missed a chance to go ahead when Yachvili's angled penalty attempt drifted wide.
The Scarlets were immediately stretched by a flowing Biarritz attack, and a scrappy first quarter ended with both sides still at the sparring stage.
Much of the entertainment came from Biarritz's travelling brass band, as neither team could find a rhythm, especially in attack. Defences dominated, and the game desperately needed some inspiration.
Biarritz almost broke through 10 minutes before the break. Yachvili put Couzinet
Llanelli had sufficient numbers to quell the attack, and number eight Scott Quinnell's ball carrying power out of defence put Biarritz in reverse gear.
But as the interval approached, Biarritz built a solid set-piece platform and drove the Scarlets over their own line.
Referee Chris White could not establish whether or not one of the Biarritz forwards had grounded possession and, after a delay, the video official agreed there was too much doubt.
Biarritz were not finished, and they demolished a Llanelli scrum before flanker Ovidiu Tonita was held up by a brilliant tackle from Scarlets wing Garan Evans.
Llanelli battled vainly to hold out, yet the visitors struck on the stroke of half-time when Couzinet crashed over from close range before Betsen was yellow-carded following a squabble with Quinnell and John Davies, and a groggy Marlu departed injured.
Biarritz took just 90 seconds after the restart to more than double their advantage.
Wing Philippe Bidabe sliced through the Scarlets' defence, found centre Martin Gaitan in support, and when the direction of attack was switched, Brusque stretched out his right arm to touch down.
Yachvili's conversion put Biarritz in charge, and left Llanelli facing a huge character test, having conceded tries either side of half-time.
The Scarlets looked devoid of ideas, meaning that Biarritz were content to play territory and keep their frustrated hosts on the back foot.
Peyrelongue's drop goal ensured that Biarritz enjoyed a healthy 12-point lead entering the final quarter, and Llanelli knew they had to conjure up something special.
Jones' converted try gave the Scarlets hope, and Biarritz showed first signs of being rattled when Brusque arrowed a kick straight into touch from outside his own 22.
It was a basic error, which allowed the Scarlets chance to at last threaten a spell of domination.
The Biarritz defence found itself stretched in all directions as Scarlets centre Mark Taylor and flanker Simon Easterby both went close to tries.
Llanelli's best efforts came to nothing and Brusque sent the disconsolate home fans away into the night with his second try, converted by Yachvili.
There was still time for replacement Bernat-Salles to add try number four, with Llanelli down and out, desperate to hear the final whistle.
- PA News