Paris - Ulster claimed their second win in two European Cup outings this season with a gritty 19-8 win at Glasgow while fellow former champions Northampton slumped to a 21-16 defeat at Castres.
Ulster, who had crushed Castres 41-17 in their opening Pool 4 match in Belfast last weekend, had led 6-3 at half-time thanks to two penalties from Paddy Jackson.
Back row forward Chris Henry extended their lead with their only try of the game after 68 minutes with Jackson adding the extras for 13-6.
Jackson added two more penalties to give 1999 champions Ulster a cushion which made Glasgow's last minute try from Fijian replacement scrum-half Nikola Matawalu a mere consolation.
"It's not easy to come here and get a victory as other teams I'm sure will find out," said Ulster head coach Mark Anscombe.
"We showed a lot of character, the boys battled their way through tough conditions against a very good team. You have got to be able to work through things because the game of rugby doesn't always go the way you want it."
Northampton, the 2000 champions, had clawed back a 15-point deficit to emerge 24-15 victors over Glasgow last week, but were outclassed by a Castres team eager to bounce back from their disastrous trip to Northern Ireland.
Castres took just six minutes to open their account when centre Paul Bonnefond crossed and Romain Teulet added the conversion.
Seven minutes later, Northampton were penalised for offside and Teulet slotted over the penalty for a 10-0 lead.
Steve Myler was on target with two quick penalties as Saints cut the deficit to 10-6.
But seven minutes from the break, lock Courtney Lawes was penalised for not rolling away and South African scrumhalf Rory Kockott made the Englishman pay with a penalty that gave Castres a 13-6 lead.
Five minutes after the interval, a scrum was reset five metres out from Northampton's line.
The ball came out to Antonie Claassen and the South African back-rower charged over for an 18-6 lead for the French side although Kockott went wide with the conversion.
On the hour mark, Samoan centre George Pisi, who was man of the match against Glasgow, brought Northampton back into the game.
He chased down a Ryan Lamb kick ahead, Kockott fumbled the ball which allowed Pisi to regather before scoring under the posts.
Myler converted for 18-13 before a cool-headed drop goal from Teulet settled Castres' nerves at 21-13.
Another Myler penalty 10 minutes from time came too late for Northampton.