Saints endured a Champions Cup shocker as Castres took them to pieces in the Stade Pierre Antoine sunshine.
Jim Mallinder’s men, who had squeezed past Montpellier in their opening game, conceded five tries in a 41-7 defeat to the rampant French outfit.
It was a sobering 80 minutes for the team from Northampton, who simply never got going and were bettered in every department.
Saints did win a string of penalties during the first half, but were unable to get any reward from kicking to the corner.
And they even conceded a try when they had a man advantage, with David Smith, who was to grab two scores, dotting down while Rory Kockott was in the sin bin.
The game then completely got away from Saints and they made a plethora of changes early in the second half, with key men replaced ahead of Friday’s game against Gloucester.
And though James Wilson scored late on, Castres continued to run amok, embarrassing their English opponents by taking the points tally past 40.
It was one of Saints’ worst European displays under Mallinder and leaves them needing a big response when the Champions Cup returns with a double-header against Leinster in December.
The opening 10 minutes had been quite nip and tuck at Stade Pierre Antoine, but Castres were to take the lead when Wilson was penalised.
Benjamin Urdapilleta landed the penalty with his accurate left foot and suddenly the home side had the bit between their teeth, led by the power of flanker Anthony Jelonch.
Saints did well to stop Castres scoring in the right corner, but they were penalised for an earlier offence and Urdapilleta doubled his team’s lead.
As expected, the Frenchmen were bringing the physicality, in attack and defence, but finally Saints managed to get a foothold in the game, winning four successive penalties in the Castres half.
With a strong wind against them, the away side continually kicked to the corner, using Harry Mallinder’s boot to earn the most territory possible.
Saints thought they had scored from a big lineout drive, but the TMO deemed there was no try and Castres eventually turned the ball over to clear.
But the home side were not finding favour with referee Ian Davies and they eventually paid the price as scrum-half Kockott was sin-binned.
However, Saints were still unable to make their territorial dominance pay and they had to scramble well to stop Castres scoring after a messy lineout gifted Alex Tulou a chance to break.
But the men in white weren’t to escape a second time as Tulou again found his way through and showed good composure to give the ball to wing Smith, who glided in for the score.
Urdapilleta converted and Castres extended their lead to 13 points, despite Kockott still being in the sin bin.
And the mouintain Saints had to climb was growing ever steeper as Castres again found a way through the defence and prop Antoine Tichit scored.
Urdapilleta put the icing on the first-half cake with a conversion that clipped the inside of the right post, and Saints headed off for half-time 20-0 down.
But the break didn’t do them any good and Castres were soon waltzing in for their third try as Smith made the most of a lovely offload and scored.
Urdapilleta rubbed salt in the Saints wounds with the successful conversion and it was now a damage limitation exercise for Mallinder’s men.
With the game gone, wholesale changes were made, with the likes of Louis Picamoles and George North taken off just 15 minutes into the second period.
Castres were hunting the bonus-point try and they would have scored it from a flowing breakaway move but for Ben Foden’s big tackle on Tulou.
Saints had to stand tall as the home side continued to come at them, and it was to be Mallinder’s men who got the next as Wilson found his way over the line.
Stephen Myler converted, but Castres responded almost immediately, bagging the bonus point they desperately wanted as replacement Alexandre Bias scored.
Julien Dumora converted to add insult to injury and there was still time for things to get worse as Antoine Dupont danced his way through the non-existent Saints defence to score.
Dumora converted again as the misery continued but it was finally over a few minutes later as Saints trudged off in front of their disappointed travelling fans.
Castres: Palis (Dumora 53); Agulla, Combezou, Ebersohn (Vialelle 69), Smith; Urdapilleta, Kockott (Dupont 61); Tichit (Lazar 60), Jenneker (Mach 56), Kotze (Montes 65); Jacquet, Moreaux (Lassalle 61); Babillot, Jelonch (Bias 56), Tulou (c).
Saints: Foden; Wilson, Hutchinson, Mallinder (Tuitavake , North (Tuala 53); Myler, Groom (Dickson 45); A Waller (E Waller 53), Hartley (Haywood 45), Brookes (Hill 53); Lawes, Paterson (Ratuniyarawa 45); Wood (c), Harrison, Picamoles (Gibson 54).
Referee: Ian Davies (Wales)