Harlequins withstood a late fightback to end Castres' unbeaten European home run and earn a narrow victory in their Heineken Champions Cup opener.
Martin Laveau's converted try reduced the deficit to two points with four minutes to play.
The hosts took the lead through Josaia Raisuqe before Louis Lynagh crossed for the English champions.
Alex Dombrandt's try handed Quins the lead and Marcus Smith's penalty put them out of reach despite Laveau's try.
Castres had not lost at home this year, while their impressive unbeaten European run at the Stade Pierre Fabre had stood since January 2015 - when...