Cardiff Blues held onto second place in the Magners League with a 30-17 win but let slip a try bonus point in the second half against Ulster.
The Blues touched down three times in the opening period and led 27-3 at the interval but the Irish side came back with two opportunist tries of their own.
Skipper Martyn Williams gave his side the best possible start with a 13th-minute try when he was on Nick Robinson's elbow after a great break by the fly-half.
Gareth Thomas and Jamie Roberts added Blues tries in quick succession and the result seemed settled by the interval.
Ulster had their best attacking spell early in the second half but Tommy Bowe spilled the ball near the Blues line.
With conditions deteriorating both sides were making handling errors but an Ulster forward drive produced a try under the posts for Ryan Caldwell.
Three minutes later Caldwell was sin-binned but the Blues failed to capitalise on their one-man advantage.
They were punished for their sloppy handling in midfield when Ulster kicked through from their own half and Bowe was awarded the touchdown for Niall O'Connor to convert.
At 27-17 Ulster were back in the game and the Blues were happy to consolidate their lead with a penalty goal from Dai Flanagan.
The game petered out in the final 10 minutes with the Blues failing to get a fourth try which would have secured the bonus point.