Munster warmed up for their Heineken Cup semi-final against Toulon with a bonus-point 32-23 victory over provincial rivals Connacht.
Edinburgh's European Rugby Champions Cup hopes suffered a major blow as they suffered a surprise 26-13 defeat to Zebre in Italy.
Round-up: Newport Gwent Dragons' European hopes took a knock as they slipped to a 45-27 defeat to Benetton Treviso.
Wales fly-half Dan Biggar kicked 20 points as Pro12 leaders Leinster went down to the Ospreys at the Liberty Stadium.
Ulster put themselves back on track in their bid to reach the RaboDirect PRO12 play-offs with a thumping 58-12 win over Connacht.
Ospreys flanker Justin Tipuric has emerged as Wales' latest potential fitness concern ahead of their tour to South Africa in June.
Scotland internationals Dougie Fife, Tom Brown and Jack Cuthbert have all signed two-year contract extensions with Edinburgh.
Glasgow climbed above Ospreys with a 29-10 victory in their rearranged clash with Benetton Treviso.
Full-back Jack Cuthbert kicked 13 points as Edinburgh secured a 23-19 triumph at Newport Gwent Dragons.
Edinburgh have announced the summer departure of three stand-offs including Leeds-bound Harry Leonard.
Scarlets had to endure a Connacht comeback before claiming a 32-30 bonus-point victory in the RaboDirect Pro12.
Glasgow Warriors leap-frogged Ospreys into fourth following an 11-9 victory at Scotstoun Stadium.
Bristol have confirmed the signing of former Wales captain Ryan Jones on a two-year contract.
John Muldoon scored two tries as Connacht beat Newport Gwent Dragons 24-8 at Rodney Parade in the RaboDirect Pro12.
Jordi Murphy scored the bonus point try in injury-time as Leinster saw off a stubborn Zebre side 27-0 to move six points clear.
Round-up: Munster beat Benetton Treviso 14-3 to maintain their 100 per cent home record in this season's RaboDirect PRO12.