Castres centre Laurent Marticorena ended Munster's nine-game winning streak and dealt their Heineken Cup quarter-final hopes a blow, kicking four crucial penalties in a 19-12 Pool 4 win at Stade Pierre-Antoine.
Boosted by eight returning Ireland internationals, Munster, having conceded a first half injury-time try to French international David Bory, drew back to 13-12 down thanks to four Ronan O'Gara kicks.
But 26-year-old Marticorena held his nerve with two late kicks to summon Castres' first win over the Irish province in four meetings.
The home side lost summer signing Brad Fleming to injury in the early stages, but Richard Dourthe's introduction on the half hour helped them to a 13-3 half-time lead.
O'Gara's excellent kicking form has been evidenced by his 89 point haul for province and country in the early season, and he pushed the visitors in front on four minutes, from 40 metres out.
Two minutes later Rob Henderson was pulled for obstruction and Marticorena kicked a right-sided reply for his first European points.
The elusive Bory, who made his international debut against Ireland in 2000, was almost put through on 11 minutes but Fleming's switch pass was ruled forward.
Fluid Castres running from former Wasps star Mark Denney nine minutes later saw Marticorena bang over his second success - this time from the left flank.
O'Gara, by this stage, was slowly coming into his own, and a lovely kick over the top from the 27-year-old saw recalled wing Mossie Lawlor almost ghost in on the loose ball.
Munster's defence, and in particular their lineout challenges on Castres skipper Remy Vigneux's ball, saw them hold firm until the final minutes of the half.
Peter Stringer had to produce another brilliant tap tackle on opposite number Alex Albouy on 37 minutes as he went on a blindside snipe, but the Castres scrum half was central to Bory's try in injury time.
He offloaded for Dourthe and Ugo Mola to link in midfield, and with Munster stretched, Bory had enough strength, when receiving on the left, to hand off Christian Cullen and the diving Lawlor to touch down in a 20 metre dash.
Marticorena converted, but Castres' indiscipline was almost their undoing.
Four minutes after the break O'Gara replied with his second penalty.
Then he punished flanker Alexandre Bias for ruck and maul offences on 51 and 56 minutes to make it a one point game.
Try as they might, Alan Gaffney's side could not overhaul the Castres lead.
O'Gara had a go from halfway on the hour, but Marticorena kicked the French to a famous win with efforts on 71 and 78 minutes.
The sides meet again next Saturday at Thomond Park, breaking the cup record for head-to-head meetings with their eighth clash.Reuse content