Paul Marshall crossed for two of the four tries in Belfast, with Ireland wing Andrew Trimble touching down for the opener as early as the ninth minute and Ruan Pienaar sealing maximum points deep into added time.

Trimble flew in on the left wing after a decisive broken-field break from full back Jared Payne to hand the hosts a confidence-boosting start after Paddy Jackson had leveled Romain Teulet's early penalty with his first contribution in a 21-point tally.

Ulster were two tries to the good by the 22nd minute as Marshall collected his own chip and chase for a fine individual score and Jackson's boot saw them 17 points up at 20-3.

Castres refused to lie down, though, and they hit back with a converted try of their own as wing Marc Andreu gathered a clever cross-field kick eight minutes before the break.

Ulster proceeded to quash any thoughts of a French revival just two minutes after the interval as Marshall snuck over from close-range to cap a fine personal display.

It was heavy going for the rest of the encounter, however, with Ulster struggling to push on for the bonus-point try and Castres adding a second score through former Gloucester and Worcester wing Marcel Garvey on 51 minutes.

Ulster had Tom Court sinbinned six minutes later and they looked like missing out on the full five points as Castres dug deep despite the game being out of reach.

But just as the crowd prepared to up sticks and wonder if three tries rather than four would be crucial at the end of the pool stages, Springbok scrum-half Pienaar marked his return from the Rugby Championship with an assured finish after Ulster attacked from deep with the final play.