Connacht secured crucial Magners League points on Friday with a rugged 16-10 win in Galway ending the Scarlets' unbeaten run.
Fly-half Ian Keatley kicked 11 points, including the conversion of Mike McComish's first half try, to drive Connacht to their first win in four league fixtures.
The Scarlets were off the boil and disappointed during a first half which saw Connacht move 13-0 ahead.
Scarlets winger Lee Williams did break through for a consolation try, which replacement Daniel Newton coolly converted for a losing bonus point.
Rhys Priestland, the Scarlets' goalkicking hero against the Dragons last week, missed his first shot at the posts as the Welshmen failed to capitalise on a decent spell of early possession.
His opposite number Keatley split the posts with a well-struck penalty and succeeded with the second of his next two attempts, a meaty 40-metre effort after the visitors were punished for an off-the-ball incident.
The Scarlets pack, minus late withdrawal Deacon Manu, played second fiddle to the hosts in the opening quarter - with the Connacht back row of John Muldoon, Johnny O'Connor and McComish swarming over the breakdown area.
Connacht were on the back foot when centre Keith Matthews was sin-binned for a late tackle - but with disciplined defence, they kept the Scarlets out of their 22 and began to force the issue up front.
When Scarlets lock Vernon Cooper was dispatched to the sin-bin, Connacht seized their chance off a powerful scrum - with Ireland call-ups Sean Cronin and Brett Wilkinson to the fore.
Scrum-half Frank Murphy broke on the blindside, and the impressive O'Connor popped an inside pass to set up athletic number eight McComish for a run to the line.
Keatley's successful conversion made it 13-0 at the break.
But the Scarlets rallied in the third quarter - and with Connacht prop Wilkinson the third player to see yellow, Priestland hammered over the resulting kick.
Full-back Morgan Stoddart, back after a seven-month injury-enforced absence, exerted little influence on proceedings - with the Scarlets backs manufacturing only a couple of notable line breaks.
Into the final quarter, the 14 men of Connacht upped the intensity - and an earth-shuddering tackle from Matthews got the hosts back in the groove.
Soon afterwards, Keatley thumped a long-range penalty over to steady the ship at 16-3 - and despite Williams' late score in the corner, Connacht stood firm for a much-needed victory.
Pens: Keatley 3
For the Scarlets:
Connacht: 15 Gavin Duffy, 14 Troy Nathan, 13 Nive Ta'auso, 12 Keith Matthews, 11 Fionn Carr, 10 Ian Keatley, 9 Frank Murphy, 8 Mike McComish, 7 Johnny O'Connor, 6 John Muldoon (capt), 5 Bernie Upton, 4 Mike McCarthy, 3 Robbie Morris, 2 Sean Cronin, 1 Brett Wilkinson.
Replacements: 16 Adrian Flavin, 17 Jamie Hagan, 18 Michael Swift, 19 Andrew Browne, 20 Conor O'Loughlin, 21 Miah Nikora, 22 Liam Bibo.
Scarlets: 15 Daniel Evans, 14 Morgan Stoddart, 13 Sean Lamont, 12 Rob Higgitt, 11 Lee Williams, 10 Rhys Priestland, 9 Tavis Knoyle, 8 David Lyons (capt), 7 Richie Pugh, 6 Simon Easterby, 5 Damian Welch, 4 Vernon Cooper, 3 Deacon Manu, 2 Ken Owens, 1 Iestyn Thomas.
Replacements: 16 Phil John, 17 Emyr Phillips, 18 Aaron Shingler, 19 Josh Turnbull, 20 Gareth Davies, 21 Dan Newton, 22 Gareth Maule.
Referee: Graham Knox (SRU)
Assistant Referees: Alan Rogan (IRFU), John Carvill (IRFU)