Northampton suffered a 21-16 Heineken Cup defeat to Castres at Stade Ernest Wallon in Toulouse on Friday.
The hosts scored two tries as they overcame their English guests to gain their first victory in this season's campaign.
These sides endured contrasting fortunes in round one with the French outfit on the wrong end of a 41-17 thrashing to Ulster and the Saints overcoming the Glasgow Warriors 24-15.
However, the hosts took a 13-6 half-time lead through Paul Bonnefond's try and penalties from Romain Teulet and Rory Kockott, with Northampton's response coming via the boot of Stephen Myler.
Antonie Claassen then went over in the second period for the home team and, although George Pisi scored for the visitors, Remi Tales' drop-goal sealed the victory.
In the previous round against Glasgow, the Midlands outfit had gone 15-0 behind before recovering to overturn that deficit. This time they went 10 points down early on.
Bonnefond touched down after a cross-field kick was superbly collected by Romain Martial. The winger then fed his team-mate who crossed the whitewash. Teulet converted and then added a penalty to extend their advantage.
But the Saints' hit back through Myler as the stand-in full-back slotted successive three-pointers to reduce the arrears.
Despite being just four points down, the visitors were finding themselves on the wrong end of Alain Rolland's whistle far too often. Another infringement saw Kockott take over the kicking duties and the scrum-half duly slotted the penalty to take Castres 13-6 ahead at the interval.
Northampton's second half didn't get off to the best start when prop Brian Mujati was sin-binned. That was to prove costly as the hosts took advantage of their dominance in the scrum when Claassen drove over from close range.
The visitors did get back into the match when George Pisi latched on to Ryan Lamb's grubber through, with Myler convertin to take the score to 18-13.
But straight from kick-off, another mistake from the away side allowed Tales to kick a drop-goal.
Myler added his third three-pointer to leave them within losing bonus-point territory, but they couldn't force the victory despite a late surge.
Tries: Bonnefond, Claassen
Pens: Kockott, Teulet
Try: G Pisi
Pens: Myler 3
Castres: 15 Romain Teulet, 14 Romain Martial, 13 Paul Bonnefond, 12 Seremaia Bai, 11 Marcel Garvey, 10 Remi Tales, 9 Rory Kockott, 1 Mihaita Lazar, 2 Brice Mach, 3 Anton Peikrishvili, 4 Iosefa Tekori, 5 Christophe Samson, 6 Jannie Bornman, 7 Piula Faasalele, 8 Antonie Claassen.
Replacments: 16 Marc-Antoine Rallier, 17 Yannick Forestier, 18 Michael Coetzee, 19 Matthias Rolland, 20 Daniel Kirkpatrick, 21 Pedrie Wannenburg, 22 Romain Cabannes, 23 Marc Andreu.
Northampton Saints: 15 Stephen Myler, 14 Ken Pisi, 13 George Pisi, 12 Dom Waldouck, 11 Vasily Artemyev, 10 Ryan Lamb, 9 Lee Dickson, 8 Rhys Oakley, 7 Tom Wood, 6 Phil Dowson, 5 Courtney Lawes, 4 Samu Manoa, 3 Brian Mujati, 2 Dylan Hartley (c), 1 Soane Tonga'uiha.
Replacements: 16 Mike Haywood, 17 Alex Waller, 18 Paul Doran Jones, 19 Mark Sorenson, 20 GJ Van Velze, 21 Martin Roberts, 22 Tom May, 23 Scott Armstrong.