Northampton's indiscipline cost them dearly as Castres ended their participation in this season's Heineken Cup with 41-22 success.
The Aviva Premiership side produced a real Jekyll and Hyde performance and were well beaten, with fly-half Stephen Myler and flanker Tom Wood both sin-binned.
The French side took full advantage, with Steve Malonga, Romain Martial (two) and Ibrahim Diarra all crossing for tries. Romain Teulet also kicked 21 points for the hosts.
Mark Sorenson, Samu Manoa and a brace from Jamie Elliott were the try-scorers for the Saints, but it was not enough.