Chris Davey's in-form Wales Under-21 side picked up their seventh consecutive win in Henley on Tuesday.
The Six Nations Grand Slam-winning side beat eighth seeds Argentina 24-18 to put one foot into the World Cup semi-finals. Their next game is against South Africa at Newbury on Saturday night.
The fourth-seeded Wales side took an early lead with tries to in-form Cardiff fly-half Nicky Robinson and Cardiff-bound winger Johnny Vaughton, but diminutive Argentinian goal-kicker Alfredo Cordone slotted three penalty goals to ensure the young Welsh side led only 17-9 at the break.
Newport prop Rhys Thomas scored early in the second half, but Cordone's fourth and fifth penalty goals narrowed the score to 24-15 to give the vocal Welsh fans some nervous moments.
The brave defence that was a highlight of Wales' opening win over Scotland came through, however, and the fourth seeds held on for a vital win. Argentina earned a bonus point with Cordone's sixth penalty goal in the 85th minute.
Wales: Tries - N Robinson, J Vaughton, R Thomas; Pen - N Robinson; Cons - N Robinson 3.
Argentina: Pens - A Cordone 6.
Halftime: Wales 17-9.