Casual defending by Cardiff Blues gifted the Scarlets a two-try start and put them on course for a bonus point 26-14 success to boost their hopes of reaching the play-offs in the RaboDirect PRO12.
Left-winger Andy Fenby was the beneficiary each time and even when the Blues cut the deficit to five points midway through the second half the visitors always looked capable of sealing their victory.
With Wales still awash with celebration after last weekend's Grand Slam and the international heroes returning to their regions, a large crowd might have been expected on a warm spring evening. But only four men involved in last Saturday's victory over France started, three of them replacements at the Millennium Stadium.
And a meagre crowd of 8,337 were present to see an error-strewn opening before Aled Thomas, a late inclusion at fly-half when Stephen Jones pulled out with illness, missed a straightforward penalty for the Scarlets.
The home side were handed a chance to break the deadlock after half an hour, but Dan Parks was similarly wayward with his penalty attempt. Jonathan Davies replaced Scott Williams in the Scarlets midfield and promptly placed a kick behind the Blues defence.
Blues wing Alex Cuthbert beat Gareth Maule to the ball, but fumbled a yard from the try-line, allowing the supporting Fenby to dot down for an easy score. It was converted by Thomas. Cuthbert made some amends when he beat Fenby to another hack downfield, this time successfully claiming the loose ball to keep the half-time margin to seven points.
But the Blues shot themselves in the foot again early in the second half, when Parks completely miskicked a rolling ball and Fenby once more had the simplest of jobs to pick up and dive over. Thomas converted superbly from the touchline.
The Cardiff side were given some hope when Sean Lamont was sin-binned for a deliberate knock-on and the Scarlets were even more generous when a careless Thomas pass was snatched from the air by Cuthbert, who raced 50 yards to the posts. Replacement fly-half Ceri Sweeney added the extras.
But the visitors were not to be denied, capitalising when a Blues move broke down, Lamont sending scrum-half Gareth Davies jinking past two defenders to touch down.
In a thrilling last quarter Blues centre Casey Laulala weaved his way over near the posts, Sweeney converting, but George North came off the bench to break two tackles and put lock Dominic Day over for the bonus-point try.