Andy Fenby's try double over Welsh rivals Cardiff Blues inspired Scarlets to a third successive Pro 12 win as they stepped up their play-off push.
Scarlets wing Fenby capitalised on an Alex Cuthbert error to touch down from Jonathan Davies' kick before making Dan Parks pay for not clearing his kick on.
Cuthbert's interception try dragged Blues into the game but Gareth Davies' score put the visitors in control.
Casey Laulala's try gave Blues hope but Dom Day's effort sealed a Scarlets win.
The Scarlets' third emphatic bonus-point victory in a row moves the Llanelli region to within a point of the play-off places and Glasgow in fourth.
The dominant four-try triumph maintains their unbeaten record against Welsh opposition this season but a third consecutive derby defeat is a blow for seventh-placed Blues' own play-off dreams.
Nigel Davies' team, however, did suffer a shoulder injury to Scott Williams and the Wales centre was replaced by Jonathan Davies 10 minutes before half-time.
And it was Davies' hopeful chip through that helped the visitors break the deadlock in what was, until that point, a tight yet compelling derby at the Cardiff City Stadium
Cuthbert, Wales' try-scoring hero in their Grand Slam clinching win over France last weekend, failed to deal with a relatively straight forward bouncing ball and Fenby pounced on his mistake to touch down from close-range.
Fenby then came close to a second score with a kick-and-chase effort with the final move of the first half.
But the 26-year-old ensured his first two-try haul of his professional career when he capitalised on more poor Blues defence to score, again from close-range.
Blues fly-half Parks miss-kicked Fenby's toe-poke forward from the base of a maul after replacement Gavin Henson was stripped of the ball under pressure in his own 22 and the qualified accountant finished off the move with consummate ease.
And as he did with Fenby's opener, Scarlets fly-half Aled Thomas, a late replacement for the poorly Stephen Jones, kicked the conversion.
The Blues stepped up the tempo and Scarlets went down to 14 men when wing Sean Lamont was sinbinned for a deliberate knock-on in front of his posts.
But it took another gift try to get the hosts going as Wales wing Cuthbert made amends for his earlier blunder to race in unopposed from halfway after intercepting Thomas' pass before Ceri Sweeney added the extras.
Lamont made up for his indiscretion as his blindside run set-up scrum-half Davies for his try.
Blues' powerful centre Laulala kept the Blues in the contest as the former New Zealander international jinked past four covering defenders to score from 10 yards out.
But the introduction of Welsh rugby's teenage sensation George North secured the victory for the Scarlets.
The wing beat three defender with a barnstorming run and when Laulala did eventually bring him down, North's lovely offload allowed Day to run in and touch down.
Cardiff Blues: Chris Czekaj (Gavin Henson, 41); Alex Cuthbert, Casey Laulala, Dafydd Hewitt, Tom James; Dan Parks (Ceri Sweeney, 51), Lloyd Williams (Richie Rees, 75); John Yapp (Sam Hobbs, 61), Ryan Tyrell (Marc Breeze, 70), Scott Andrews (Nathan Trevett, 75), Macualey Cook, James Down (Maama Molitika, 53), Michael Paterson (Cory Hill, 75), Josh Navidi, Xavier Rush (c).
Scarlets: Liam Williams; Sean Lamont, Gareth Maule (c) (George North, 70), Scott Williams (Jonathan Davies, 32), Andy Fenby, Aled Thomas, Gareth Davies; Rhodri Jones (Phil John, 66), Ken Owens (Matthew Rees, 59), Deacon Manu (Peter Edwards, 58), Dominic Day, Damian Welch (Sione Timani, 53), Josh Turnbull, Johnathan Edwards, Ben Morgan (Kieran Murphy, 37).
Replacements not used: Liam Davies.
Referee: Nigel Owens (Wales)
Assistant Referees: Neil Hennessy, Jon Mason
Citing Commissioner: Dennis Jones
TMO: Gareth Simmonds