La Rochelle threw Pool Five wide open as they became the first side in 18 games to win at Gloucester.
In a match marred by indiscipline, the French visitors prevailed 24-18.
Smarting from their 13-6 home defeat to the Cherry and Whites last weekend, La Rochelle shocked their Aviva Premiership Rugby opponents, for whom even two tries from Lesley Vainikolo could not save the day.
A fiery second half saw the visitors lose both Alexandre Bares and Tamato Leupolu to the sin bin, while Vincent Roux was sent off for picking up two yellow cards.
The home side dominated territory but did not convert their chances and once number eight Thomas Soucaze powered his way over, they were chasing the game.
Benjamin Dambielle collected a 14-point haul and prop Jonathan Garcia also touched down for La Rochelle.
Vainikolo crashed over either side of the break and Freddie Burns crossed for a late try in a frantic final 10 minutes but it came too late.