Paul Bonnefond and Antonie Claassen scored tries for the hosts, with George Pisi's late effort not enough to secure a second straight win for Northampton following success against Glasgow last time out.

The French side enjoyed a dream start as Bonnefond touched down with just seven-and-a-half minutes on the clock. Remi Tales' cross-field kick wasn't the most accurate but right wing Romain Martial leapt brilliantly to claim the high ball, before surging forward and offloading to the onrushing 22-year-old French international.

Castres' prolific goal kicker Romain Teulet added the extras from the 15-metre line on the right and the full back stretched the lead to 10 points before the quarter-of-an-hour mark after Saints second row Courtney Lawes was penalised for playing the ball off his feet.

Northampton hit back straight from the restart, though, as Stephen Myler made Castres pay for sealing off their own ball on the edge of the 22. And the Saints were back to within a single score before the game was 25 minutes old as Myler slotted a second penalty to make it 13-6.

Castres came close to adding a second try to their tally just before the half-time whistle but Iosefa Tekori's score was ruled out thanks to a flying tackle from Ken Pisi that dragged Marcel Garvey's foot into touch.

The home side began the second period in impressive fashion as Bonnefond almost struck again only for an unfortunate bounce to prevent him from collecting his own chip ahead after Seremaia Bai's beautiful pass out of the tackle.

Saints prop Brian Mujati then saw yellow for pulling down a maul on 44 minutes when his side lost their own lineout ball just 10 metres from their tryline and, even though Rory Kockott missed the resulting penalty, Castres did reap the rewards soon after. Huge pressure on the visiting scrum eventually told when No8 Claassen dotted down just as referee Alain Rolland called for Castres to 'use it'.

Kockott failed with the conversion attempt but Castres were still 18-6 up and supposedly cruising before George Pisi scored Saints' first and only try of the game on 59 minutes to give the 2000 Heineken Cup winners hope.

Pisi pounced after Kockott fumbled Ryan Lamb's clever kick ahead as Saints made the most of stolen lineout ball on the opposition 10-metre line.

Myler's conversion from directly beneath the sticks brought his side to within five points but a Tales drop goal made it 21-13 just moments later. Myler's 73rd-minute penalty led to a nervy ending for Castres but they deservedly hung on to record a first Heineken Cup victory since last December's win over the same opposition.