Scarlets held off Castres to make an encouraging start to their Heineken Cup campaign at Parc y Scarlets.
A tight first half saw the home side lead 9-6 thanks to three Stephen Jones penalties, with Romain Teulet striking two for Castres.
The game burst into life as Sean Lamont, Ben Morgan and Jonathan Davies claimed tries for the Scarlets.
Marc Andreu and Chris Masoe replied for the French and it needed a late Rhys Priestland kick to settle Welsh nerves.
Crucially the 80th-minute penalty from Priestland, on for Wales fly-half rival Jones, took Scarlets eight points clear.
That denied Castres a losing bonus point and could yet prove crucial in the final Pool 1 standings.
Despite the return to action of their World Cup stars, the Scarlets were fortunate to lead 9-6 at the break against a bigger Castres pack who dominated at the scrum and looked dangerous at the line-out.
Even with a seven-man pack when number eight Joe Tekori was sin-binned for a late tackle on Scott Williams, Castres had the upper hand in the set-piece.
The Scarlets looked more comfortable when the ball was moved wide, with wign George North a potent threst, but let two golden try opportunities go begging through forward passes.
But Castres also butchered glorious scoring opportunities and had nearly taken a dramatic lead from the kick-off only for flanker Steve Malonga to drop the ball over the line.
The French side took the lead on two minutes with a penalty for full-back Teulet before Jones replied with two successful attempts.
Full-back Teulet kicked his second just two minutes before Tekori found himself in the bin and Jones slotted his third penalty just before the break.
The Scarlets began the second half with real intent and scored the opening try with Castres still down to 14 men, sending Lamont away under the posts.
Teulet got a third penalty back before the Scarlets scored a length-of-the-field try with number eight Morgan crashing over after North had made the long yards.
But the Scarlets were not able to relax as Castres wing Andreu went in at the left corner. Teulet's conversion reduced the Scarlets' lead to 23-16.
A third try for the hosts arrived 13 minutes later, however, when Scott Williams raced up the left flank and gave a try-scoring pass to Davies.
Castres skipper Masoe muscled his way over to give Scarlets some nervous moments, but the Welsh region had the final say with Priestland putting over a last-minute penalty.
Scarlets: L Williams; North, S Williams, J Davies, Lamont; S Jones, Knoyle; I Thomas, M Rees, R Thomas, Day, Welch, Shingler, J Edwards, Morgan, Timani.
Replacements: Evans for North (75), Priestland for S Jones (54), R Williams for Knoyle (54), John for I Thomas (61), Owens for M Rees (61), R Jones for R Thomas (67), Timani for Welch (16), Pugh for Timani (80).
Castres: Teulet; Nicolas, Bonnefond, Cabannes, Andreu; Tales, Lacrampe; Peikrishvili, Rallier, Ducalcon, Rolland, Capo, Masoe, Malonga, Tekori.
Replacements: Garcia for Cabannes (43), Bernard for Tales (72), Sanchou for Lacrampe (64), Coetzee for Peikrishvili (66), Mach for Rallier (47), Wihongi for Ducalcon (56), Murray for Rolland (72), Bornman for Malonga (53).
Referee: Greg Garner (RFU)