Newport Gwent Dragons stunned a strong Ospreys side to go level on points with them and edge closer to qualification for next season's Heineken Cup.
Tries from Will Harries and Matthew Watkins helped the Dragons into a healthy 21-10 first-half lead.
The Ospreys did reply through James Hook and Andrew Bishop.
But Aled Brew had already made sure there would be no way back for their Welsh rivals after bursting through to claim the hosts' third try.
The Dragons stay sixth in the Magners League, but join the Ospreys on 33 points, 10 points ahead of the Scarlets who are the fourth-placed Welsh region.
The win was no less than they deserved after coming charging out of the blocks and they were gifted three points inside 90 seconds when the Ospreys were penalised for going over the top, Jason Tovey kicking.
And it was not long before they claimed their first try thanks to Tovey's cross-field kick which saw Harries go over in the corner for an unconverted try.
The Ospreys were in danger of losing composure and captain Ryan Jones saw a penalty against his side brought forward 10 metres for complaining enabling Tovey to extend the Dragons lead to 11-0.
But that lead was immediately cut by three points as the hosts were guilty of failing to release, enabling Dan Biggar to kick.
It settled the Ospreys nerves and they grew as the half went on, and it took excellent defending to keep them out.
But the visitors were patient and finally made the breakthrough 10 minutes before the break when Hook went over in the corner and Biggar converted superbly to bring them within a point.
The Dragons were then dealt a blow with the loss of flanker Dan Lydiate to an ankle injury, but extended their lead with another penalty when Adam Jones was caught offside.
That four-point gap could have been brought back to one when the Dragons were caught offside five metres from their own line, but the Ospreys opted for a scrum instead and wasted their chance.
And it cost them as just before the break Watkins burst through the visitors defence to sprint home from 30 metres side-stepping Lee Byrne on the way.
Tovey converted to round off an excellent first half for Paul Turner's side who went in with a 21-10 lead.
And they went is search of further points after the restart with the Ospreys try-line insight, but had to do some defending of their own and allowed the Ospreys back into it for going over the top at the ruck, following Byrne's kick and chase.
But the Dragons were determined not to be pegged back and were rewarded on 56 minutes when Brew breached the Ospreys defence to go over after patient build-up.
The Ospreys seemed to come alive after that and they attacked relentlessly in the last 20 minutes, grabbing a converted try through Bishop after good work by Nikki Walker.
But it came too late for them to push for a losing bonus point.
Dragons coach Paul Turner:
"It was a great win for us and to put it in perspective, we've beaten a good side here.
"It's not about Paul Turner, it's about the coaching staff who've worked really hard last week, especially the players.
"I thought to a man we were outstanding. We had little blips in our performance, but we fully deserved the win."
Ospreys forwards coach Jonathan Humphreys: "It's a little frustrating, we go away for a couple of months then come back and it can be disjointed.
"But take nothing away from the Dragons, I thought they were outstanding.
"I thought they were very, very worthy winners tonight.
"We've got a huge local derby next week and hopefully we will perform better than we did tonight."
Newport-Gwent D'gons: M. Thomas, Harries, Watkins, Smith, Brew, Tovey, W. Evans, Gustafson, Willis, Palmer, Sidoli, Charteris, Lydiate, G. Thomas, Falatau. Replacements: Riley for Smith (64), Arlidge for Brew (60), Castle for Palmer (48), Coombs for Lydiate (32). Not Used: S. Jones, MacDonald, Leadbeater.
Ospreys: Byrne, Bowe, Bishop, Hook, Walker, Biggar, M. Phillips, James, Bennett, A. Jones, J. Thomas, Evans, Collins, Holah, R. Jones. Replacements: Owen for Bishop (79), Parker for Biggar (58), Nutbrown for M. Phillips (74), Hibbard for Bennett (51), Bevington for A. Jones (75), Gough for Evans (58), Tiatia for Holah (74).
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