Fiji winger Setariki Tuicuvu scored twice as Toulon beat Castres 41-19 on Saturday to take the lead in the French Top 14.
Tuicuvu, 28, was left out of his country's Rugby World Cup squad earlier this year but claimed his first tries of the season at Stade Mayol to send his club above Stade Francais.
His fellow winger Gael Drean, who was playing in the fifth tier of French rugby two years ago, also crossed on two occasions for 14-man Toulon after centre Jeremy Sinzelle was sent off early in the second half.
A much-changed Castres, runners-up in 2022, are in fourth after their third defeat in eight games this term.
A minute's silence will be held before each game at all levels of the sport in France this weekend after a 16-year-old player named Thomas was killed during a night out last week.
Later on Saturday, controversial former French rugby federation president and ex-France head coach Bernard Laporte takes charge of struggling Montpellier against Oyonnax for the first time after he was appointed director of rugby last week.
In December, former sports minister Laporte was handed a two-year suspended prison sentence for his financial dealings with Montpellier owner Mohed Altrad.
He was also banned from holding any rugby post for two years, but that is on hold pending his appeal.
Elsewhere, Stade Francais can go back top as they head to surprise packages Pau, who are also eyeing first place, and France captain Antoine Dupont starts as champions Toulouse host Clermont.
On Sunday, South Africa's two-time World Cup winning captain Siya Kolisi is expected to make his Racing 92 debut against Ronan O'Gara's La Rochelle.