James sparked the winning try with a counter-attack
Scarlets: (13) 20
Tries: MacLeod, Easterby
Cons: S Jones (2)
Pens: S Jones (2)
Toulouse: (12) 19
Tries: Clerc (2), Poitrenaud
Cons: Courrent (2)
Llanelli Scarlets hung on to claim a famous Heineken Cup win over three-time champions Toulouse at Stradey Park.
An early onslaught by the home side gave them a 13-0 lead through Scott MacLeod's try and Stephen Jones' boot.
But Toulouse powered back with tries from Vincent Clerc (2) and Clement Poitrenaud for a 19-13 lead.
A Dafydd James counter-attack set up a fine try for Simon Easterby, converted by Jones, as an off-form Gaffie du Toit blew late winning chances for Toulouse.
The Scarlets travel to the Stade Ernest Wallon next week with a 100%, three-game record in the tournament.
Toulouse will be in danger of an early exit after two defeats, although they at least take a bonus point from their trip to Wales.
We will travel with confidence to Toulouse
Scarlets fly-half Stephen Jones
The Llanelli side had torn into the visiting superstars from kick-off, determined to reverse their four pool losses to Toulouse from their previous two European campaigns.
A sustained attack was kept out by superb defence that forced a five-metre line-out, but Toulouse captain Yannick Bru blew his throw in to give Scottish lock MacLeod a simple, third-minute try.
Jones converted and - with Yannick Nyanga in the sin bin for a reckless challenge on James - added two penalties.
Nyanga's return sparked a comeback, though, the Toulouse flanker feeding Clerc.
The France wing had looked his side's best player and easily skinned Scarlets full-back Ceiron Thomas on the outside for a fine score.
Powerful ball-carrying from Thierry Dusatoir then paved the way for Poitrenaud's try, Valentin Courrent's conversion making it 13-12 to the home side at half-time.
Clerc's try brace could not save Toulouse
A careless Scarlets line-out saw their advantage wiped out straight after the break.
A powerful driving maul gave superb ball to Yannick Jauzion, who crossed the line but had possession punched out of his hands by the defence.
The ball fell to Clerc, who picked up for a simple try, converted by Courrent.
The Llanelli side were shell-shocked, but Thomas responded by counter-attacking from a deep Toulouse kick.
He fed man-of-the-match James, whose block-busting run was carried on by Alix Popham.
The number eight found back-row colleague Easterby who crossed for a superb try, converted by Jones to give his side a one-point advantage.
With Courrent off to a blood-bin injury, du Toit - who had struggled at outside-half all game - missed a 30m penalty that would have restored his side's lead.
As the tension mounted with the French encamped in the home 22 in the closing minutes, du Toit blew a drop-goal chance, before a desperate, failed drop-kick from Clerc sealed the visitors' fate.
Scarlets outside-half Stephen Jones:
"We talked about making an impact from the start of the game, but then we dropped off for 40 minutes, although at half-time we still felt we could beat them.
"At 20-19 up, it was a game of cat and mouse out there and we were fortunate they missed a couple of drop-goal attempts at the end to win it.
"It is very important to win your home games, and we will travel with confidence to Toulouse next weekend."
Scarlets: C. Thomas, James, King, G. Evans, M. Jones, S. Jones, Peel, I. Thomas, Rees, Manu, Cooper, MacLeod, Easterby, G. Thomas, Popham.
Replacements: Daniels for M. Jones (52), Dunlea for Manu (75), D. Jones for G. Thomas (61). Not Used: Owen, A. Jones, Stuart-Smith, B. Davies.
Toulouse: Poitrenaud, Clerc, Fritz, Jauzion, Heymans, Du Toit, Courrent, Human, Bru, Poux, Pelous, Albacete, Nyanga, Dusatoir, F. Maka.
Replacements: Medard for Poitrenaud (70), Kunavore for Courrent (52), Hasan for Human (72), Lacombe for Bru (79), Millochlusky for Pelous (52), Bouihlou for Nyanga (52), Lamboley for F. Maka (72).
Sin Bin: Nyanga (11).
Referee: Alain Rolland (Ireland).