Toulon's Top 14 play-off hopes took a step backwards on Friday when they went down 18-12 in a try-less encounter in Castres.
The showdown between the championship's top two place-kickers lived up to it's billing as Castres full-back Romain Teulet and Toulon fly-half Jonny Wilkinson provided all the points from the tee.
Wilkinson added his fourth penalty of the evening with the very last kick of the game to save a losing bonus point for Toulon.
The result sees Castres maintain their perfect record at home this year. They will finish the weekend in the top three in the standings, seven points ahead of Toulon, who are now seven but could drop as low as ninth by the end of Round 22.
Only six teams qualify for the play-offs and with just four rounds to play, RCT find themselves in a tight spot.
The hosts led from start to finish as Teulet opened the scoring in the first minute and continues to slip the uprights to give CO a 15-6 lead at half time.
Former Wales centre Gavin Henson had hoped to make his debut for Toulon but will have to wait another week as he was not yet fit enough and did not make the substitutes' bench.
Pens: Teulet 6
Cons: Wilkinson 4
Yellow card:Chesney (Toulon - 21st min)
Castres:15 Romain Teulet, 14 Marc Andreu, 13 Josh Tatupu, 12 Pierre-Manuel Garcia, 11 Yoan Audrin, 10 Seremai Bai, 9 Alexandre Albouy, 8 Iosefa Tekori, 7 Yannick Caballero 6 Chris Masoe (C), 5 Rodrigo Capo Ortega, 4Matthias Rolland 3 Luc Ducalcona , 2 Benjamin Kayser, 1 Yannick Forestier.
Replacements: 16 Carl Hoeft, 17 Mathieu Bonello , 18 Scott Murray, 19 Ibrahim Diarra, 20 Thomas Sanchou , 21 Steve Malonga, 22 Pierre Bernard, 23 Danie Saayman.
Toulon:15 Rory Lamont, 14 Christian Loamanu, 13 Gabriele Lovobalavu, 12 Felipe Contepomi, 11 Rudi Wulf, 10 Jonny Wilkinson, 9 Pierre Mignoni, 8 Juan Fernandez Lobbe, 7 George Smith, 6 Joe van Niekerk (c) 5 Kris Chesney, 4 Dean Schofield, 3 Davit Kubriashvili, 2 Michael Ivaldi, 1 Laurent Emmanuelli.
Replacements:16 Jean-Charles Orioli, 17 Carl Hayman, 18 Christophe Samson, 19 Olivier Missoup, 20 Benjamin Lapeyre, 21 Fabien Cibray, 22 Geoffroy Messina, 23 Saimone Taumoepeau.