Woeful display costs Castres against relegation candidates
Castres will go into the back-to-back games against Munster on the back of their worst display of the season.
The French champions were lethargic and paid the price when Agen struck for a late try to win 16-13 in Castres' backyard, Stade Pierre Fabre.
Agen, second from bottom, deserved their win as they were the better side throughout and dealt with everything Castres had to throw at them.
An ill-disciplined Castres display saw three players binned in the second half and having gone to the corner with three kickable penalties early in the game, coming away empty-handed each time, they paid the price when Agen snatched it at the death.
The defeat sees Castres slip to seventh in the Top 14 and with a lot of ground to make up to get back in contention.
Elsewhere, Exeter Chiefs fell to their first defeat in the Premiership this season when they went down 28-26 away to Harlequins.
Rob Baxter's Chiefs stay in second place, two points adrift of champions Saracens.
Gloucester are in third with their fifth win in nine league matches after running out impressive 36-16 winners over Worcester Warriors at Kingsholm.
International Rugby Newsletter