The Scarlets boosted their PRO12 play-off hopes by sealing a rare league double over Ulster with a 21-20 victory at the Kingspan Stadium on Sunday, and in doing so traded places with their hosts in the table.
The hard-earned result - which saw the lead change hands several times in the opening 40 minutes - saw Wayne Pivac's men move up to third while Les Kiss' Ulster, who began the weekend at the top, slipped back to fourth after claiming a losing bonus point and falling to only their second home league loss this term.
Two first-half tries from Scarlets full-back Michael Collins helped the Welsh side take the hugely valuable win, with fly-half Aled Thomas adding a conversion and three penalties.
Ulster led 17-15 at half-time after superb tries from Craig Gilroy and Luke Marshall both converted by Paddy Jackson, who - on his debut as captain - also went on to add a penalty and a drop-goal for 10 of the home side's final total, but it was not enough to clinch the win on Roger Wilson's 200th appearance for the province.
The second period was a much edgier affair, with only three penalties being scored and a yellow card being shown to Chris Henry.
A great line break from skipper Hadleigh Parkes led to an early try for the visitors, with the centre drawing Stuart Olding to put full-back Collins over on the left after just four minutes.
Thomas was wide with the difficult conversion but the visitors were up and running. They then stretched their lead in the 14th minute when Thomas bisected the posts with a close-range penalty with Ulster infringing at a breakdown near their line.
The home side quickly responded two minutes later when, with a penalty coming their way after a Jacob Stockdale burst, Jackson slotted a drop-goal.
Four minutes later and Gilroy struck from Marshall's loopy pass, with the winger setting off from his own half before chipping Collins and regathering the ball to score a superb try in the right corner to tie the scores.
Jackson hit a brilliant conversion to put Ulster in front for the first time, but their lead only lasted three minutes as the Scarlets came straight back down the field with Collins again crossing out on the left.
Thomas then emulated Jackson with an excellently struck conversion from the touchline and the Scarlets led 15-10.
Then with four minutes remaining the Marshalls combined, with Paul sensing a mismatch at the side of a ruck and bursting through to feed Luke who, with a lot of work to do, beat three defenders to just make the right corner. The television match official was consulted, and awarded the score.
Again, that tied matters and, once more, Jackson nailed a superb conversion from the right touchline to nudge Ulster in front again at 17-15.
As with the pattern of the game up to that point, the visitors came back and, after John Andrew was penalised, Thomas had a chance with the last play of the half to put the Scarlets in front again only for him to hit the upright and see the ball bounce out.
Thomas made amends six minutes after the restart with a penalty under the posts after wave after wave of Scarlets ball-carriers had failed to get over the line and that 18-17 lead became 21-17 when Thomas slotted another penalty two minutes later after Chris Henry was yellow-carded for killing the ball.
Ulster managed to avoid conceding any further points during Henry's absence and closed the Scarlets' lead to a point with a 63rd-minute penalty from Jackson.
Surprisingly, neither side scored again and the Welsh region left the Kingspan with a vital four points.