London - Last season's runners-up Northampton Saints will not go beyond the pool phase in the Heineken Cup this time after suffering their third successive defeat, 41-22 at Castres on Saturday.
Northampton had England flanker Tom Wood and flyhalf Steve Myler sin-binned in each half as Romain Martial scored two tries and Steve Malonga and Ibrahim Diarra one each for the French side, who had also lost their first two games in Pool 1.
Munster took charge of the pool as Ireland flyhalf Ronan O'Gara celebrated his 100th Heineken Cup appearance with 12 points in the Irish side's 17-14 away win over the Scarlets.
Munster's 13th successive win over the Welsh side put them on 12 points to the 10 of the Scarlets, who they host next week.
There was something of an upset in Pool Five as Treviso beat 2010 runners-up but currently struggling Biarritz 30-26 thanks to tries by Michele Rizzo, Robert Barbieri, Cornelius van Zyl and the goalkicking of Kristopher Burton.
Biarritz have seven points and Treviso six but English Premiership champions Saracens went top on 10 points after beating the Ospreys 31-26 at Wembley later on Saturday.
Harlequins' record 14-match unbeaten run was brought to a halt on Friday night when they went down 21-10 at home against four-times champions Toulouse in the battle of the English and French league leaders.
A try in each half by Fiji winger Timoci Matanavou and solid goalkicking by Luke McAlister had Toulouse in control throughout, with Mike Brown's try merely producing a better scoreline for the outclassed Londoners.
"The right side won," said Quins' director of rugby Conor O'Shea. "They were incredibly physical and stopped us on the gain line - that was a lesson for us and our crowd have just seen a great, great European side."
The victory put Toulouse in control of Pool 6 with three wins out of three ahead of their home return against Harlequins next week.
Gloucester remain in the hunt as they beat Connacht 14-10 away for their first win.
Cardiff also have three wins out of three in Pool 2 after flyhalf Dan Parks kicked 20 points, including two drop goals, in their 25-8 home thrashing of Edinburgh, who had also won their first two.
Ulster duly beat Italian outsiders Aironi, collecting a bonus point as they scored five tries in a 31-10 win in Pool Four, though Leicester and Clermont, who meet on Sunday, will still have a lot to say about who progresses.
European Cup results on Saturday:
Castres 41 Northampton 22
Scarlets 14 Munster 17
Racing Metro 14 London Irish 34
Treviso 30 Biarritz 26
Saracens 31 Ospreys 26
Connacht 10 Gloucester 14
Played on Friday
Cardiff Blues 25 Edinburgh 8
Ulster 31 Aironi 10
Harlequins 10 Toulouse 21
Playing on Sunday (kick-offs in GMT)
Bath v Leinster (12:45)
Glasgow v Montpellier (12:45)
Clermont v Leicester (15:00)