Saints slipped to a 21-16 defeat to Castres in soggy conditions at the Stade Ernest-Wallon in Toulouse.
A late George Pisi try was enough to secure a losing bonus point that could eventually prove crucial in a tight pool four, but it was the French side who claimed the victory.
Tries from Paul Bonnefond and Antoine Claassen, plus points from the boots of Romain Teulet, Rory Kockott and Remi Tales were enough for the win.
Saints replied through Pisi’s second-half score and 11 points from the boot of Stephen Myler, but that only proved enough for a single point as the sin-binning of Brian Mujati hindered their chances of a win.
With Ulster winning in Glasgow, Saints now sit second in the pool, with the double-header against the Irish side in December set to prove pivotal.
With the wind and rain whistling around Toulouse in the lead up to the game, how both sides dealt with the conditions was going to be key.
The ball resembled a bar of soap as it slipped through Castres hands early on, but they juggled it well to score the first try of the game.
Iosefa Tekori kept the ball in play brilliantly and after Tales kicked the ball crossfield, Bonneford was handed the chance to dance through the away side’s backline.
Teulet added the extras and it was the worst possible start for Jim Mallinder’s men, 7-0 down inside eight minutes.
Teulet then added to the damage as Saints conceded their third penalty inside the first 14 minutes and the full-back sent the penalty sailing through the posts.
Northampton needed a response, and as a chorus of ‘come on you Saints’ started up, the away fans got their wish as Myler, starting at full-back, slotted a penalty.
And Saints steady Eddie was on the mark again minutes later as Alain Rolland handed the men in green, black and gold another penalty.
The soggy conditions were taking their toll and after Ryan Lamb fumbled and knocked on, Castres put the squeeze on in the scrum to claim a penalty, which Teulet fluffed.
Rolland was hammering Saints on the penalty count and skipper Dylan Hartley got a warning after Castres were awarded yet another shot at goal. This time, Kockott stepped up, and made no mistake.
Saints were struggling to assert themselves and the French were looking to send them to the guillotine before the break.
And they nearly did, as Marcel Garvey was just nudged into touch by Ken Pisi, before he offloaded to skipper Tekori, who would have scored.
Castres started the second half as they did the first, on the front foot, and their dominance resulted in Mujati making the error of pulling down a maul in the 45th minute.
He was promptly sin-binned, putting his side under severe pressure and gifting the home team another penalty, which Kockott passed up, floating it to the left of the upright.
However, the French side’s appetite for points was soon satisfied as they pulled Saints to piece in a scrum five metres out and No.8 Claassen was the grateful recipient to put Castres 18-6 up.
With Mujati back from the bin, Northampton required a spell of possession to ease the pressure and they almost got themselves back in the game, as Ken Pisi was tapped on his way to the try line.
But Ken’s brother George was to have more luck moments later as Lamb’s kick through was fumbled by the home side and the Samoan centre flicked it forward with his own boot before dotting down.
Myler converted and Saints were back to within five, but that was to last just seconds as they made a mess of catching the kick-off, allowing Tales to grab the ball and land a drop goal.
Lamb then tried to a copycat effort but his attempt was woefully short, and wide, before Myler made a better job of a penalty to get his side back in losing bonus point territory at 21-16.
Saints battled right the way to the end but couldn’t get over the line and a forward pass ended their hopes of a draw, or even victory.
Castres Olympique: Teulet (Andreu 51); Martial, Bonnefond, Bai (Cabannes 65), Garvey; Tales (Fitzpatrick 76), Kockott; Lazar (Forestier 62), Mach (Rallier 68), Peikrishvili (Coetzee 53), Tekori (Rolland 57), Samson, Bornman, Faasalele (Wannenburg 70), Claassen.
Northampton: Myler; K Pisi, G Pisi, Waldouck, Artemyev; Lamb, Dickson (Roberts 65); Tonga’uiha (Waller 68), Hartley, Mujati (Doran-Jones 68), Manoa (Sorenson 65), Lawes, Dowson, Wood, Oakley (van Velze 62).
Replacements not used: Haywood, May, Armstrong.
Referee: Alain Rolland (Ireland)