Ken Owens marked his first game as Scarlets captain with a try in his side's 29-24 win over Gloucester.
Centre Scott Williams added another as Scarlets led 17-10 at half-time, with Henry Trinder scoring for the visitors.
A Michael Tagicakibau try after the break extended the lead before the English side hit back through Aleki Lutui.
Rory Pitman crashed over for Scarlets but Steve McColl's score for Gloucester ensured a nervy finish.
James Hook, playing in Wales for the first time since signing for Gloucester from Perpignan, scored one penalty and conversion at Parc y Scarlets.
The West Wales side get their Pro12 season underway on 6 September at home to Ulster, while the visitors open their Aviva Premiership campaign against Northampton Saints on 5 September.