The Belfast side have a perfect record in the Pool and quarter-final qualification is basically theirs to lose as they look to stretch their unbeaten tournament record to four games with a second victory over the Saints in the space of a week.
The visitors were in irrepressible form against one of the top sides in the English Premiership and crossed for four tries through Andrew Trimble, Tommy Bowe, Jared Payne and Dan Tuohy.
Ulster, who were last season's losing finalists to Leinster at Twickenham and are unbeaten in this season's PRO12, will already have one eye on a quarter-final place.
The Ulstermen were crowned Kings of Europe back in 1999 and have been a regular fixture in the last eight of the tournament for the last two seasons and already look like one of the favourites to do the same this season.
Castres Olympique are now second in the table and move ahead of the Saints after their narrow 9-6 victory over Glasgow Warriors in Scotland. The Scottish club are still looking for their first win and are bottom of the Pool.
The French side will be looking for a far more comfortable victory in the second leg of their double-header on their home patch against the Warriors to keep the pressure on the Irish province.