A clinical second-half performance form the Scarlets was enough to claim the win, built on the back of a string of impressive performances from their International contingent.
It was the visitors however who started the brighter building a platform through their dominant pack.
The first period was all about the boot, Stephen Jones kicking three penalties to Romain Teulet’s two as the Welsh region went into the break 9 v 6 ahead.
The second period was a much different story at Parc y Scarlets and the Scarlets signalled their intentions early on with a Sean Lamont try.
Teulet grabbed a penalty back for the Scarlets before North powered upfield setting up his side’s second try as number eight Ben Morgan touched down.
Marc Andreu grabbed a try back for Castres and Teulet’s conversion reduced the scores to 23 v 16.
The Scarlets though were soon back on form, Scott Williams racing clear before feeding Jonathan Davies to score.
Castres captain Chris Masoe set up a tense finish with his late try but Rhys Priestland came off the bench for the hsots to land a crucial late penalty which also denied Castres a losing bonus point.
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