Cardiff Blues South U18 came out on top in the inter-regional derby at Sardis Road on Tuesday, clinching a 0-20 victory.
Gareth Nicholas' side led 0-15 at the interval before Teddy Williams' second half try put the game beyond doubt.
Both sides made a host of changes for the clash, with Cardiff Blues North making eight changes from their defeat to Scarlets last week, while Cardiff Blues South made 13 due to a short turn-around from Sunday's win over RGC.
South were forced to make a late change to their announced line-up, as Jacob Beetham was called up at outside half to take the place of Ed Lewis.
And it was Beetham who opened the scoring, making no mistakes from a straight-forward penalty in front of the sticks with nine minutes on the clock.
It didn't take long for the visitors to extend their lead. Captain James Bartlett stole possession inside the North's half and set off down the right, before finding Jay Jones on his inside, who ran in unopposed.
South captalised from another breakdown to cross for their second try. Quick thinking from Joe Patterson saw the wing pick up and go, finding space on the fringe of a ruck and stepping the final man to dive over in the corner.
Cardiff Blues North were reduced to 14 men before half time as George Thomas was sent to the sin-bin for tackling off the ball.
The hosts were shown a second yellow card in the second half, as Cian Lewis Hughes deemed to have sacked a lifter, and the visitors took advantage of the extra man in the pack, with Teddy Williams crashing his way over from close range, making it 0-20.
Cardiff Blues North travel to Ystrad Mynach to face Dragons on Tuesday, January 16, while Cardiff Blues South visit Ospreys on Wednesday, January 17.
Cardiff Blues South's defence coach, Josh Turnbull, was delighted with his side's victory in the derby, and has been pleased to the the whole squad playing a part in the wins over RGC and Cardiff Blues North.
"As a coaching team, we were delighted that the boys got the win, and we always knew we would have to be at our best to come up to Sardis Road and face a good Cardiff Blues North side," said Turnbull.
"There were a lot of changes made from Sunday's win over RGC. That was always our plan and it's been great to be able to give game time to the whole squad.
"As a defence coach, it's always good to keep the opposition's score to zero and the boys have taken what we've been working hard on in training over the last few weeks.
"The focus now turns to our next fixture away to Ospreys next Wednesday."