Scarlets climbed to third in the Pro12 after edging Ulster 21-20 away from home on Sunday thanks to two first-half tries from Michael Collins and the boot of Aled Thomas.
Full-back Collins crossed in the fourth minute at Kingspan Stadium, getting over in the right corner following a missed pass from Ulster's Darren Cave.
Thomas missed the conversion, but sent over a penalty in the 14th minute to see the visitors go 8-0 up in Belfast.
Ulster hit back through a Paddy Jackson drop goal and Craig Gilroy's converted try, the winger scoring a brilliant solo effort in the 20th minute by racing into opposition territory and gathering his own kick.
Jackson added the extras to make it 10-8 in the hosts' favour.
They soon repeated their early mistake, though, affording Scarlets a man advantage out wide for Collins to bag his second try, Thomas converting in the 24th minute.
Ulster centre Luke Marshall needed the television match official to confirm his try in the corner 10 minutes later, the effort converted by Jackson, as Thomas missed a penalty to see the home team go in 17-15 ahead at the break.
A low-scoring second half meant the match was decided by two Thomas penalties, the second after the hosts' flanker Chris Henry was sent to the sin bin for playing the ball on the ground.
Ulster managed only one penalty in reply through Jackson to suffer a narrow home defeat which left them fourth, four points behind leaders Connacht.