London Irish were dragged deeper into the relegation mire after champions Harlequins ground out a 26-15 victory in front of 82,000 fans at Twickenham.
The Exiles were in touch for 78 minutes but left empty-handed after a late penalty try from Harlequins' dominant pack denied them a losing bonus point.
Harlequins had taken the lead after a scrappy first half with a try from scrum-half Danny Care, and Nick Evans added 16 points with the boot.
Ian Humphreys kicked all the points for London Irish, who are now just one point ahead of the Aviva Premiership's bottom side Sale Sharks.
A brilliant try from scrum-half Joe Simpson was the highlight of London Wasps' 34-15 bonus-point victory over London Welsh.
Simpson had an outstanding game as Wasps came from behind with tries from wing Elliot Daly, replacement prop Will Taylor and, right at the end, a fourth try for wing Tom Varndell.
Welsh fly-half Gordon Ross kicked all his side's points with five penalties, while opposite number Nicky Robinson landed three of his own, Daly booted one and replacement stand-off Stephen Jones kicked the only conversion of the match.
The conversation of a penalty try three minutes from time denied Exeter their first Premiership victory over west country rivals Bath as the visitors fought back to secure a 12-12 draw at Sandy Park.
Fly-half Gareth Steenson kicked four penalties for Exeter, while Bath responded with a Michael Claassens try and then, in the last five minutes, won a penalty inside the opposition's 22.
They opted to kick for the corner and the Chiefs were then penalised at the ruck. Bath repeated the move and referee David Rose awarded a penalty try, with Tom Heathcote adding the extras.