Treviso marked their first Magners League appearance with an historic 34-28 win over the Scarlets at Stadio Comunale di Monigo on Saturday.
The Italians trailed 22-9 at half-time but produced a stunning performance in the second period to make an instant impact on the competition.
Scrum-half Tobie Botes contributed 21 points including a try and Italy international Ezio Galon and Tommaso Benvenuti also crossed.
Botes landed an early penalty but that by no means set the tone for the first half as the Welsh side opened up a significant lead.
Fly-half Rhys Priestland converted his own try in the sixth minute and, after Chris Burton's drop-kick for Treviso, Scarlets wing George North crossed to make it 12-6 after 16 minutes.
Another Botes penalty kept the hosts in touch but Priestland responded in kind and a converted try from North right on half-time left the visitors seemingly well in control.
Treviso had lost number eight Manoa Vosawai to a yellow card two minutes earlier but the extra man failed to help the Scarlets as their opponents came out firing at the start of the second half.
Botes again landed an early kick at goal before going over for his try just at the end of Vosawai's 10-minute absence.
He missed the conversion but, after a raft of substitutions from both teams, the hosts levelled thanks to centre Galon's try, Botes' goal putting them two points in front at 24-22.
He extended that margin with a penalty but two successful shots from Priestland put the Scarlets a point ahead at 28-27 with 10 minutes remaining and left the hosts staring at heartbreak after their heroic second-half efforts.
There was to be one final twist in the tale though as, with four minutes remaining, wing Benvenuti crossed and Botes kicked the extras to earn a 34-28 win.
Tries: Botes, Galon, Benvenuti
Con: Botes 2
Pen: Botes 3
For the Scarlets:
Tries: Priestland, North 2
Pen: Priestland 3
Treviso: 15 Luke McLean, 14 Tommaso Benvenuti, 13 Ezio Galon, 12 Alberto Sgarbi, 11 Benjamin De Jager, 10 Khristopher Burton, 9 Tobias Botes, 8 Manoa Vosawai, 7 Alessandro Zanni, 6 Robert Barbieri, 5 Cornelius Van Zyl, 4 Antonio Pavanello (capt), 3 Pedro Di Santo, 2 Leonardo Ghiraldini, 1 Augusto Allori.
Replacements: 16 Diego Vidal, 17 Lorenzo Cittadini, 18 Ignacio Fernandez Rouyet, 19 Enrico Pavanello, 20 Paul Derbyshire, 21 Fabio Semenzato, 22 Simon Picone, 23 Brendan Williams.
Scarlets: 15 Morgan Stoddart, 14 George North, 13 Gareth Maule, 12 Regan King, 11 Andy Fenby, 10 Rhys Priestland, 9 Martin Roberts, 8 Rob McCusker (capt), 7 Johnathan Edwards, 6 Josh Turnbull, 5 Damian Welch, 4 Lou Reed, 3 Deacon Manu, 2 Emyr Phillips, 1 Iestyn Thomas.
Replacements: 16 Ken Owens, 17 Phil John, 18 Peter Edwards, 19 Vernon Cooper, 20 Jonny Fa'amatuainu, 21 Tavis Knoyle, 22 Daniel Evans, 23 Jon Davies.
Referee: David Wilkinson (Ireland)