Harlequins conceded seven second-half tries against Castres at Stade Pierre Antoine to crash to the worst Heineken Cup loss in their history.
HEINEKEN CUP POOL FOUR
Tries: Capo-Ortega 2, Froment 2, Vigneaux, Albouy, Bernad, Mola
Cons: Martoicorena 6
Pens: Marticorena 2
Tries: Duffy, Sherriff
The visitors started well, scoring first with an Andy Dunne penalty, and trailed only 11-8 after 50 minutes.
But the last half an hour saw Castres queuing up to score, Remi Vigneaux opening the floodgates before Alexandre Albouy and Romain Froment touched down.
Harlequins have only pride to play for when they face Munster on 15 January.
Castres: Metcalfe; Raffault, Marticorena, Denney, Mola; Delaigue, Albouy; Reggiardo, Vigneaux, Galasso, Nallet, Spanghero, Bernad, R Capo Ortega, Volley.
Replacements: Argenese, F Capo Ortega, Deen, Froment, Dourthe, X Sadourny, Morlaes.
Harlequins: Duffy, Harder, James, Deane, Keogh, Dunne, So'oialo, Jones, Tiatia, Dawson, Rudzki, Miall, Molitika, Sherriff (capt), Hortsmann.
Replacements: Dott, Muggeridge, Easter, Diprose, Staunton, Reay, Williams.