Grenoble condemned Toulouse to back-to-back defeats in the space of seven days with a 15-6 victory over the champions at Stade Lesdiguières.
In other results on Saturday, there were home wins for Stade Francais, Castres, Bayonne and Perpignan, while leaders Toulon won 15-9 at Agen to go seven points clear.
Grenoble 15-6 Toulouse
No tries were scored in the opening game of the day as five penalties from Valentin Courrent saw him come back to haunt his former side as they beat Toulouse 15-6.
The former Toulouse scrum-half was on target with three shots at goal in the first-half to Nicolas Bézy's two before two more from Courrent sealed the points for the home side.
Stade Francais 36-23 Biarritz
A try apiece from Paul Sackey and Francis Fainifo along with points from the boot of Jerome Porical, Julian Dupuy and Jules Plisson saw the Parisians seal a fine win at Stade Charlety.
Biarritz's points came from Taku Ngwenya and Jean-Pascal Barraque, whose tries were back up by ten points from Dimitri Yachvili and a late conversion from Julian Peyrelongue.
Victory sees Stade Francais move level on points with BO.
Bayonne 39-13 Mont de Marsan
It was a five-star performance from Bayonne as they demolished basement side Mont-de-Marsan on Saturday, with tries coming from Scott Spedding, Manu Ahotaeiloa, Guillaume Bernad, Mike Phillips and Sam Gerber. Benjamin Boyet kicked the remaining 14 points.
The win lifts the hosts above Racing-Metro and up to ninth position in the standings.
Castres 31-10 Racing Metro
Seremaia Bai and Romain Martial both claimed a brace of tries as Castres followed up their back-to-back Heineken Cup wins over Glasgow with a Top 14 success over Racing-Metro.
In a one-sided spectacle, the visitors could only manage a penalty try and Gaëtan Germain's five kicked points in a result that sees Castres now move level with third-place Toulouse.
Perpignan 30-19 Montpellier
Perpignan gave their play-off hopes a huge shot in the arm after overcoming Montpellier in a showing that saw Wales international James Hook score 20 points for the Catalan side.
Home tries were scored by Florian Cazenave and Sébastien Taofifenua while Montpol responded with a Juan Figallo score and 14 points from the boot of Benoît Paillaugue.
Agen 9-15 Toulon
Jonny Wilkinson kicked all of his side's points, with four penalties and a drop-goal as high-flying Toulon recorded their eleventh success of the Top 14 season at Stade Armandie.
With Toulouse having lost to Grenoble at Stade Lesdiguières earlier in the day, the result extends Toulon's lead at the top to seven points as they continue their fine start to 2012/13.