Scarlets were overwhelmed by French league leaders Bordeaux-Begles in the Heineken Champions Cup.
Teenager Louis Bielle-Biarrey scored a hat-trick with further tries from Cameron Woki, Geoffrey Cros, Maxime Lamothe and Nans Ducuing completing a seven-try demolition.
Bordeaux were inspired by Argentina wing Santiago Cordero, who played out of position at scrum-half.
Scarlets responded with tries from Liam Williams and Gareth Davies.
The defeat completed another miserable European weekend for the four Welsh regions with a clean sweep of losses.
Welsh teams have lost all nine games played in the Champions Cup and Challenge Cup so far this season.
Scarlets and Ospreys prop up each pool in the top tier, while Dragons are languishing in their group in the second-string competition.
Scarlets, Cardiff and Ospreys will be battling to stay in Europe next weekend in the final round of Champions Cup group games.
It was Scarlets' first match in the tournament after the first two fixtures were called off because of Covid 19-related issues.
The opening-round match at Bristol was forfeited and marked as a 28-0 defeat when they were unable to fulfil the fixture because of quarantine restrictions after returning from an aborted United Rugby Championship trip to South Africa.
The home game against Bordeaux was called off because of Covid travelling restrictions.
That meant this was only the Scarlets' second game since 22 October, which was painfully obvious during a disjointed display.
Scarlets made eight changes from the 1 January win over the Ospreys, including the return of captain Jonathan Davies and British and Irish Lions prop Wyn Jones.
There was late disruption with Scotland flanker Blade Thomson and Wales prop WillGriff John ruled out, replaced by Morgan Jones and Samson Lee.
Bordeaux were also forced into late changes by three positive Covid tests, with half-backs Francois Trinh-Duc and Maxime Lucu dropping out of the original selection.
Argentina wing Cordero switched to scrum-half to partner France fly-half Matthieu Jalibert and the hosts adapted much more impressively.
A dropped high ball from Wales full-back Williams led to the opening try for France flanker Woki after clever interplay between Jalibert and Cordero.
The makeshift scrum-half turned creator again when he set up 18-year-old full-back Bielle-Biarrey for a try in only his second match.
Williams' forgettable first half continued when he was shown a yellow card for a professional foul, with wing Cros taking advantage with the hosts' third try.
Scarlets' failing lineout and struggling scrum ensured they had no forward platform as they trailed 21-0 at half-time.
Hooker Ryan Elias was held up over the line early in the second half before Bielle-Biarrey scored his second and the Bordeaux bonus-point try following some slick offloading.
Replacement fly-half Rhys Patchell then provided a clever chip kick which allowed Liam Williams to pounce and score.
Scarlets were unable to repeat the trick when Cordero latched on to a loose kick and set up a try for replacement hooker Lamothe.
Scrum-half Gareth Davies responded for Scarlets with a trademark interception try before replacement Ducuing dived over for Bordeaux's sixth score.
Bielle-Biarrey completed his hat-trick before Cordero had a try disallowed for obstruction, while Scarlets replacement Phil Price was shown a yellow card for persistent scrum penalties.
Scarlets head coach Dwayne Peel said: "We are disappointed. The reality is we did not respect the ball and turned over possession too cheaply and put ourselves under pressure.
"For large periods we held them up for as long as we could but offensively, especially for five minutes in the first half, we put some good sets together but unfortunately we let it slip.
"If we can't keep hold of the ball, you put yourself under that sustained pressure. We have to look at ourselves in the mirror and get better. We were not good enough today."
Bordeaux Begles: L Bielle-Biarrey; G Cros, U Seuteni, Y Falatea-Moefana, N Hulleu; M Jalibert, S Cordero; T Paiva, J Dweba , V Cobilas, T Jolmes, J Marais (capt), C Woki, B Vergnes, A Roussel.
Replacements: Lamothe, Poirot, Tameifuna, Douglas, Bochaton, Dubie, Mori, Ducuing.
Scarlets: L Williams; J McNicholl, J Davies (capt), S Williams, S Evans; S Costelow, G Davies; W Jones, R Elias, S Lee, S Lousi, M Jones, A Shingler, T Lezana, S Kalamafoni.
Replacements: M Jones, P Price, H O'Connor, J Price, C Tuipulotu, D Blacker, R Patchell, I Nicholas
Referee: Karl Dickson (England)
Assistant referees: Mike Hudson & John Meredith (England)
TMO: Tom Foley (England)