The English side produced a real Jekyll and Hyde performance and were well beaten, with fly-half Steve Myler and flanker Tom Wood both sin-binned in the first and second half.
The French side took full advantage and the English side could not survive playing with 14 men for 20 minutes in this must-win Pool One clash.
Saints coach Jim Mallinder will now have to focus on his side's Aviva Premiership campaign and will know he has some work to do to rebuild the battered confidence and morale of his side.
The visitors could not have made a worse start, with Castres scoring their first try after only three minutes when impressive openside flanker Steve Malonga finished off a sweeping attacking move.
Northampton replied with a trademark forward surge from short range when lock Mark Sorenson burrowed his way over and Myler added the extras to level the scores.
The metronomic boot of Castres cult figure Romain Teulet put the French side back ahead with two penalties, before the English side rallied with a well-taken try by wing Jamie Elliott.
Myler was sin-binned for a second high tackle just before half-time and 14-man Saints conceded a second try almost immediately after a delightful chip to the corner by number eight Chris Masoe put wing Romain Martial in at the corner to make it 23-12 to Castres at half-time.
It was the Saints who scored next, as Elliott ran from inside his own half after he intercepted Martial's pass.
American number eight Samu Manoa, who impressed with his power and athleticism, then clawed the Saints back to within four points when he scored his side's fourth try for a bonus point midway through the second half.
But Tom Wood's yellow card proved costly for the Saints and they could not cope going down to 14 men for a second time. Castres wing Martial and flanker Ibrahim Diarra took full advantage and crossed for tries to record a bonus point and an emphatic victory.
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