Toulon lost on home soil for the first time since January when they were beaten 28-24 by Stade Français.
The reigning Top 14 and European champions paid the price for poor goal-kicking in the second half, and an underperforming scrum, but it was a fully deserved result for Stade who showed a lot of ambition in their game.
Stade went over twice in the first half, through Alexandre Flanquart and Julien Arias, but Toulon came back after the break with tries from Xavier Chiocci and Drew Mitchell.
But Jules Plisson kept the scoreboard ticking over for Stade, including a late drop goal, and they were well worth their win.
Heinke van der Merwe started at loosehead prop for the visitors, while Toulon had Juan Smith at openside flank and Michael Claassens at scrumhalf.
Meanwhile, Julien Bardy's second-half try gave Clermont a first-ever win at Ernest Wallon as Toulouse lost a third straight game 13-9.
It was the first time during Guy Noves's reign that Toulouse had lost three on the trot.
Round 5 results:
La Rochelle 41 Castres 16
Toulouse 9 Clermont 13
Bayonne 23 Brive 6
Oyonnax 40 Grenoble 27
Racing Métro 28 Lyon 11
Toulon 24 Stade Français 28
Bordeaux-Bègles v Montpellier
Photo: Boris Horvat/AFP Photo