The Ospreys' hopes of becoming just the second Welsh side to win in France in the Heineken Cup were shattered by Castres at the Stade Pierre Antoine.
HEINEKEN CUP POOL 4
Castres (23) 38
Tries: Denney, Volley, Lhande, Pen
Cons: Delaigue (3)
Pens: Delaigue (2)
DGs: Delaigue, Sadourny
Ospreys (7) 17
Tries: A Jones, Spice
Cons: Henson (2)
First-half tries from Mark Denney and Paul Volley helped Castres to a 23-7 lead over the Celtic League leaders.
The onslaught continued after the break, a Benjamin Lhande score and a penalty try securing a bonus point.
To compound the Ospreys' misery, Wales forward Jon Thomas was sent off in the final minute for punching.
Tries for Adam Jones and Jason Spice were scant consolation for the Welsh as they lost their 100% season record.
In a furious start by the French, former Wasps centre Denney made the midfield break for the first try after 18 minutes.
Mario Ledesma and Alexandre Albouy did well in the build-up before the ball came back to Denney to score in the corner.
Straight after the restart, David Barrier led a huge forward drive for Castres, another ex-Wasps man Volley managing to
get his hands on the ball to crash over for the second try.
The stunned Ospreys were in danger of going to pieces, losing tight-heads at the scrum and being broken almost at will.
Centre Sonny Parker's defensive efforts helped to hold them together, and his 38th-minute burst from half-way got them back
into the game.
Andy Lloyd took the ball on, before it came to prop Jones who barrelled over from short range.
The Welsh side's new-found composure was rocked on the stroke of half-time, though, as Yannick Delaigue fired over a snap
drop-goal from 45m.
The Ospreys held a driving maul from the French early in the second half, but the livewire Albouy spotted a gap down the
He put Lhande into space, and the wing was able to step Adrian Durston for the third try.
The visitors' plight got more desperate with second-row Lyndon Bateman sin-binned for cynically pulling back Castres
full-back Ugo Mola off the ball.
The French turned the screw in the scrummage, and a series of re-set scrums under the posts resulted in a penalty try, giving
Castres a bonus point.
Gavin Henson helped restore some pride with a half-break, freeing Spice for the Ospreys' second try.
But replacement Xavier Sadourny rubbed salt into the visitors' wounds with a late drop goal, and in the game's last action Thomas saw red after lashing out at Castres prop Mauricio Reggiardo after the Welshman had been held back.
The result completed a miserable weekend for the Welsh sides in Europe, all four of their teams falling to defeats.
Ospreys scrum-half Spice denied suggestions that his side were not ready for the step up in intensity that comes in the Heineken Cup.
"We knew what we were in for and came here with high expectations, but we just didn't play well and made too many mistakes," he told BBC Sport Wales.
"They played better, tidier rugby and profited from our errors. We just have to put it right against Munster next week."
Castres: U Mola; B Fleming, R Dourthe, M Denney, B Lhande; Y Delaigue, A Albouy; J
Fitzpatrick, M Ledesma (capt), A Galasso, D Barrier, N Spanghero, R Froment, P Volley, A Bias.
Replacements: G Arganese, M Reggiardo, R Capo Ortega, G Bernad, N Morlaes, X Sadourney, N Raffault.
Ospreys: A Durston; R Mustoes, D Bishop, S Parker, S Williams; G Henson, J Spice; D Jones, B Williams (capt), A Jones, B Cockbain, L Bateman, A Lloyd, R Pugh, J Thomas.
Replacements: M Davies, A Millward, P James, A Newman, J Bater, A Williams, M Jones.
Referee: Dave Pearson (England)