Northampton Saints have slipped to a 21-16 defeat away to Castres in the second round of the Heineken Cup.
The result puts the Saints in a difficult position to ensure automatic qualification from the group.
Paul Bonnefond opened the scoring for the French side with a try after seven minutes.
Romain Teulet and Steven Myler added to the scoreline with an exchange of penalties to put the half-time score at 13-6.
Northampton's Brian Mujati was given a yellow card early in the second half and Castres took full advantage, adding another try through captain Antoine Classen.
George Pisi pulled the Saints back within reach with a score before Castres re-established their lead through a drop goal.
Saints pushed to get something out of the game and although Stephen Myler secured a bonus point with a penalty, Northampton could not find a winning try.
