Montpellier scrum-half Benoit Paillaugue thrived in what was a low-scoring affair, and he traded early points with Toulouse fly-half Lionel Beauxis.
Winger Timoci Nagusa put the home side in control with a try that was converted by Paillaugue, and despite a late fightback from Toulouse, it was not enough to overhaul their opponents.
The defeat dents the defending champions' hopes of booking a place in the semi-finals, while the win cements Montpellier's place in the play-off picture.
Stade Francais secured a fine 28-20 victory over struggling Agen at the Stade Armandie, while in another basement battle, Bayonne edged out Grenoble 31-24 in a seven-try thriller at the Stade Jean Dauger.
In the other match on Saturday, Racing Metro raced away to a 34-9 victory over Mont-de-Marsan.