Northampton conceded six tries as they fell to a 41-7 Pool 4 defeat by Castres in the Champions Cup in France.
The Saints beat Montpellier in their opener last week, but were outgunned by Castres’ forwards, and victims of their own sloppiness in possession.
Castres impressed throughout, with Samoan wing David Smith crossing twice.
Northampton winger James Wilson scored a late consolation, but Alexandre Bias and Antoine Dupont crossed to secure the bonus point for the hosts.
The home side pinned the Saints in their own half early on,capitalising with a pair of penalties from Argentina fly-half Benjamin Urdapilleta.
Castres’ scrum dominated, but a series of infringements at the breakdown saw scrum-half Rory Kockott sin-binned in the 22nd minute.
However, that failed to halt Castres’ momentum, and when number eight Alex Tulou produced a trademark barnstorming break, Smith was on hand to finish under the posts.
More first-half punishment came when Kockott, not long returned to the pitch, cleverly found space with a chipped kick, before prop Antoine Tichit bundled over the try line.
Northampton needed to start the second half well to have any hope of salvaging points, but it was Castres who continued their dominance, and Smith scored his second in the corner within three minutes of the restart.
The match became scrappy as the second half progressed, but opened up in the final 10 minutes. First, Smith made his only error of the game when sweeping at the back, knocking on and allowing Wilson to cross down his flank for Northampton.
Straight from the restart, Castres rampaged forward with renewed intent, and it was replacement Bias who stormed over to secure the bonus point for the French.
And more excellent work from Smith allowed scrum-half Dupont to round off a sparkling performance from Castres.
Castres: Geoffrey Palis; Horacio Agulla, Thomas Combezou, Robert Ebersohn, David Smith; Benjamin Urdapilleta, Rory Kockott; Antoine Tichit, Jody Jennifer, Daniel Kotze, Loic Jacquet, Victor Moreaux, Mathieu Babillot, Anthony Jelonch, Alex Tulou.
Replacements: Brice Mach, Mihita Lazar, Yohan Montes, Thibault Lassale, Alexandre Bias, Antoine Dupont, Julien Dimora, Florian Vialelle.
Northampton Saints: Ben Foden; James Wilson, Rory Hutchinson, Harry Mallinder, George North; Stephen Myler, Nic Groom; Alex Waller, Dylan Hartley, Kieran Brookes, Courtney Lawes, Michael Paterson, Tom Wood, Teimana Harrison, Louis Picamoles.
Replacements: Mikey Haywood, Ethan Waller, Paul Hill, Api Ratuniyarawa, Jamie Gibson, Lee Dickson, Nafi Tuitavake, Ahsee Tuala.