Connacht continued their fine record at Stradey Park with a shock win at the home of the reigning champions.
Scarlets (13) 21
Try: Peel, D Jones
Pens: Bowen (3)
Connacht (7) 29
Tries: O'Loughlin, Robinson, McPhillips, Swift
Cons: Warwick (2), Elwood
Pen: M McHugh
A Dwayne Peel try helped the Scarlets to an early 10-0 lead, but Connacht tries either side of half-time fell to Conor O'Loughlin and Ted Robinson.
Conor McPhillips got another, before a controversial Dafydd Jones try handed the lead back to the home side.
A 50m Mark McHugh penalty put Connacht back ahead, and Mike Swift's last-minute try won them a bonus point.
Gareth Bowen opened the scoring with a second-minute penalty in a furious start by the Scarlets.
Scott Quinnell took a quick free kick near half-way to set off another attack, as the Llanelli side quickly recycled in a
The ball came to Matthew Watkins in midfield who showed good strength to off-load to Peel, the scrum-half scampering
over for the first try.
But the gritty visitors dug in, and as the break approached they began to hammer at the Scarlets' line.
Scrum-half O'Loughlin took advantage when Watkins spilled ball on the floor, gathering to squeeze through a melee for his
side's first score.
The try was controversial as O'Loughlin appeared to lose control of the ball before grounding, but referee Andrew Ireland had
The Scarlets' worries were eased as Bowen added his second penalty on the stroke of half-time to make it 13-7.
But their lead lasted just five minutes as Connacht came out with the wind at their backs.
A missed touch kick from Garan Evans fell to wing Robinson who showed great pace to cut in from the right and score under
The Scarlets failed to capitalise on a superb start
Centre James Downey then split the Scarlet midfield defence with a powerful run, the recycled ball finding left wing
McPhillips who had the pace and power to score in the corner.
A Bowen penalty hauled the Scarlets back, before their forwards exploded from a line-out 10m from the Connacht line.
As their driving wedge scattered the visitors' defence, referee Ireland lifted his arm to award a try to Dafydd Jones.
The score was bitterly contested by Connacht as there appeared to be doubt over whether Jones grounded the ball.
That gave the Scarlets a two-point lead, but it lasted barely five minutes before McHugh boomed over a 50m penalty for
Connacht's joy was complete as flanker Mike Swift crossed from close range with the final move of the game, securing a bonus point and condemning the Scarlets to their fourth defeat of the campaign.
"We knew it would be incredibly difficult here and so it proved, it was a very tight finish," said delighted Connacht centre McHugh.
"The Scarlets shouldn't worry, they have so much quality in their side that they'll put things right before the Heineken Cup."
Scarlets full-back Garan Evans added: "They are a very good team who are hard to play against and they gave an outstanding performance, but we are hugely disappointed with ourselves."
Scarlets: G Evans; T Selley, M Watkins, M Taylor, S Finau; G Bowen, D Peel; I Thomas, M Rees, J Davies, V Cooper (capt), A Jones, D Jones, G Thomas, S Quinnell.
Replacements: P John, C Hawkins, C Wyatt, J Mills, M Phillips, A Thomas, R Jones.
Connacht: D Slemen; T Robinson, J Downey, M McHugh, C McPhillips; P Warwick, C O'Loughlin; R Hogan, J Fogarty, S Knoop, P Myburgh, A Farley, M Swift, M Lacey, M Carroll.
Replacements: H Bourke, A Clarke, C Short, J Muldoon, M Walls, E Elwood, C Keane.
Referee: Andrew Ireland (SRU).