Munster secured qualification for the Heineken Cup quarter-finals by beating Castres at Thomond Park to top Pool 1.
Munster led 16-7 at half-time thanks to a Johne Murphy try and three penalties and a conversion from the boot of Irish fly-half Ronan O'Gara.
Pierre Bernard converted a try by Toulouse flanker Yannick Caballero.
After the break Bernard kicked one penalty and O'Gara two, then Wian du Preez scored Munster's second try of the game, with O'Gara converting.
Murphy's eighth-minute try set the Pool One leaders on their way, with Caballero hitting back for the French outfit.
Although Castres had nothing but pride to play for, their understrength side ensured a frustrating second half for the home faithful until du Preez touched down with 12 minutes to go.
Unforced errors and missed scoring chances made for a poor display from Munster, who will need to up their game at Northampton in round six as they chase a home draw in the last eight.
Munster set about their task with a couple of surges from man of the match Donnacha Ryan and James Couglan almost leading to an early try.
But Castres could only watch as a training ground move at a lineout, involving Ryan, Conor Murray and Murphy sent the latter through a weak tackle from Thierry Lacrampe and over for a converted score.
Bernard missed a kickable penalty after BJ Botha infringed at a scrum, but the Munster set piece gave very little away after that.
O'Gara, now the tournament's most capped player with 102 appearances, fired over the first of his penalty goals in the 16th minute.
But Castres enjoyed a good chunk of first half possession and they silenced the big crowd with a try following a break from Ibrahim Diarra and Lacrampe's quick tap.
Munster were caught cold as a quick thread of passes ended with Saimone Taumoepeau putting flanker Caballero past Lifeimi Mafi's challenge and over to the left of the posts.
Bernard added the extras for a 10-7 scoreline, however Munster had a firmer grasp on the second quarter with O'Gara providing direction and captain Paul O'Connell impressing in the air and at the breakdown.
Two more penalties from O'Gara stretched the lead back out to nine points by the break, with some defiant defending from the French side keeping Munster at bay late on.
Bernard, with a crisply-struck penalty from long range, reduced the deficit to 16-10 early in the second half.
O'Gara struck the right post when trying to reply at the opposite end, before a break from his half-back partner Murray put Castres back on the defensive.
The visitors kept O'Gara out close to their posts, but a solid scrum led to the Cork man punting into touch as their try-scoring search intensified.
Castres' commitment in the tackle saw them hold out and Munster had to settle for another three points from O'Gara, who slotted over from a central position.
Murray had a try ruled out for blocking from O'Connell and despite having scrum half Lacrampe sin-binned for persistent infringements, Castres' try-line remained intact.
That was until Mafi found some space in midfield, getting the hosts enough momentum to send South African loosehead du Preez piling over from a few metres out.
Munster: D Hurley; J Murphy, K Earls, L Mafi, S Zebo; R O'Gara, C Murray; W du Preez, D Varley, BJ Botha; D Ryan, P O'Connell capt; P O'Mahony, N Ronan, J Coughlan.
Replacements: D Fogarty for Varley (73), M Horan for du Preez (76), S Archer for Botha (74), Donncha O'Callaghan for Ronan (14), B Holland for O'Mahony (78), T O'Leary for Murray (66), I Keatley for O'Gara (74), D Barnes for Mafi (78).
Castres: R Martial; V Inigo, P Bonnefond, PM Garcia, PG Lakafia; P Bernard,T Lacrampe; S Taumoepeau, MA Rallier, A Peikrishvili; M Rolland, R Capo Ortega; Y Caballero, S Malonga, J Bornman.
Replacements: M Bonello for Rallier (66), M Coetzee for Taumoepeau (56), K Wihongi for Peikrishvili (56), S Murray, R Kockott for Lacrampe (69), S Bai, M Nicolas for Inigo (73), Roussel for Caballero (73)
Not used: S Murray, S Bai
Referee: Andrew Small (RFU)