Cardiff Blues 21-14 Ulster
Playing his first game since retiring from international rugby, former Scotland fly-half Dan Parks scored 16 points, including a try, with winger Tom James claiming the hosts' other touchdown as they came from 11-6 down at the break.
Three penalties from Ruan Pienaar and a try from Robbie Diack made up Ulster's scoring.
This was the second successive game the Blues had moved from the Cardiff City Stadium to their old home at the Arms Park and last week's 8,000 attendance for the defeat of Connacht was followed by an estimated 9,000 capacity crowd turning up for the visit of the Ulstermen.
Opsreys 23-7 Aironi Rugby
The Ospreys began life under new head coach Steve Tandy with a hard-fought 23-7 RaboDirect PRO12 victory over Italian side Aironi at the Liberty Stadium.
Tries at either end of the match - one a penalty touchdown and the other from centre Andrew Bishop - plus three penalties and a conversion by outside-half Dan Biggar and another two points from Matthew Morgan gave the Ospreys the win.
Aironi's only score came via a converted try by wing Sinoti Sinoti.
Leinster 16-13 Scarlets
Fergus McFadden kicked a last-minute penalty to complete a hard-fought 16-13 victory for Leinster and deny the Scarlets a share of the spoils in Dublin.
McFadden coolly slotted the kick from outside the 10-metre line as Leinster kept on course for a home semi-final berth in the RaboDirect PRO12.
Aiming to put themselves back in the play-off picture, the Scarlets impressed in the first half with a Dan Newton try putting them 10-3 in front at half-time.
McFadden's early second-half try got Leinster level and Newton and McFadden swapped penalties, before the Ireland international had the final say.