Scarlets virtually ended Wasps' Heineken Cup hopes with a vital Pool Six win at Stradey Park.
HEINEKEN CUP POOL SIX
Scarlets (3) 10
Tries: James, M. Jones
Pens: Bowen (3)
Wasps (6) 13
Con: Van Gisbergen
Pens: King (2)
Wasps led 6-3 after a dull first-half, but Dafydd James put the home side ahead when he picked up Alex King's loose pass to score under the posts.
Mark Jones added a superb try from the halfway line, but Wasps replacement Tom Rees hit back three minutes later.
Gareth Bowen gave the Scarlets some breathing space with two late penalties after Wasps had missed three attempts.
The defeat means the English champions are rooted to the bottom of the table with just three points from three games.
Scarlets leapfrog Edinburgh to second place with eight points, three adrift of leaders Toulouse.
The home side well worth their victory at a foggy Stradey Park as they made Wasps look a very ordinary side.
There was a touch of fortune in the Scarlets' opening try when James picked up King's midfield to become the joint top try-scorer in the competition's history.
But the second try on 64 minutes was all down to the power and speed of winger Jones who broke from the halfway line and showed a clean pair of heels to Wasps full-back Mark van Gisbergen.
Sensing the gravity of the situation, Wasps finally came to life and Rees capitalised on good work from half-backs Matt Dawson and King to touch down unopposed.
Van Gisbergen's conversion brought them firmly back in contention at 15-13 down, but the Scarlets finished stronger and Bowen's late penalties denied Wasps a losing bonus point.
Scarlets director of rugby Gareth Jenkins:
"Toulouse are still favourites, let's be realistic about that, and we've now got to go to Wasps next week.
"I thought John Davies was rightly man of the match, and he has earned the respect of everyone who would have watched that game today.
"He is unbreakable. The scrum wasn't a problem for us, and John is still a player that deserves all the respect he's got."
Wasps rugby director Ian McGeechan:
"We were nowhere near the standard we want to be at and our worst two away performances this season have been in the Heineken Cup.
"We are really annoyed with ourselves, but we will be full-on in the next three European games and we intend to hurt a few people if we can.
"The game was nowhere near the pace we would have liked, and we gave all away. You cannot expect to win games if you do that."
Scarlets: Byrne; G Evans, Watkins, D James, M Jones; Bowen, Peel; Williams, Rees, J Davies; Louw, A Jones; Easterby, G Thomas, A Popham.
Replacements: Wyatt for Easterby (65).
Not Used: R Davies, Gravelle, Afeaki, Stuart-Smith, Hercus, B Davies.
Wasps: Lewsey; Sackey, Erinle, Abbott, Voyce; King, M Dawson; Payne, Ibanez, J Dawson; Shaw, R Birkett; Worsley, O'Connor, Dallaglio.
Replacements: Van Gisbergen for Abbott (61), Reddan for M Dawson (80), Barratt for Ibanez (74), Rees for Worsley (63).
Not Used: McKenzie, Skivington, Hoadley.
Ref: A Rolland (Ireland).