The Ospreys Under-18 side lost their 100 per cent record in the WRU Age Grade Championship after a home defeat against Cardiff Blues on Wednesday night.
Despite trailing 6-0 inside 13 minutes, the visitors ran out 21-6 winners at Llandarcy courtesy of tries from no.8 Reuban Tucker and left wing Harry Robinson.
Matthew Morgan, who helped himself to 22 points in the Eyasses 37-12 victory in North Wales two weeks ago, kicked two penalties early on to suggest that it would be the home side who made it three successive victories in the championship this term.
However, Blues outside half Joseff Griffin halved the deficit after 19 minutes and on the half hour, Tucker nipped in for a try Griffin failed to improve.
The excellent pivot made up for that miss with a second penalty, a minute later, and two minutes before the interval, Robinson crossed for his first try of the season.
Griffin converted and added a penalty 10 minutes into the second half to make sure it was the Blues who remain unbeaten.