Castres outscored Pau four tries to three in a 30-21 win on Saturday in the rescheduled Challenge Cup fixture.
The bonus point win puts Castres four points off top spot behind Newport Gwent Dragons and Sale Sharks with two rounds to go in Pool 2.
Pau opted to rest star signings Colin Slade and Conrad Smith but briefly took the lead at the start of the second half thanks to a try from Julien Fumat.
Castres however rallied and sped out to a comfortable 30-14 lead, until Sean Dougall crossed late on for the hosts.
An early penalty from Geoffrey Palis was followed up by a David Smith try to give the visitors a fast start.
Matters went from bad to worse for Pau with the sin-binning of Thomas Bianchin, Palis slotting a second penalty to make it 13-0.
Bianchin, back from the bin, scored for Pau just before half-time with Brandon Fajardo's conversion closing the gap.
Fumat's converted try then put Pau ahead before they were back down to 14 men after a yellow card for Jale Vatubua.
Castres responded by scoring two tries in three minutes from Sitiveni Sivivatu and Florian Vialelle before replacement hooker Julien Beziat added the bonus point score in the 75th minute.
Tries: Bianchin, Fumat, Dougall
Cons: Fajardo 3
Yellow Card: Bianchin, Vatubua
Tries: Smith, Sivivatu, Vialelle, Beziat
Cons: Palis 2
Pens: Palis 2
Pau: 15 Romain Buros, 14 Marika Vunibaka, 13 Julien Fumat, 12 Jale Vatubua, 11 Mathieu Acebes, 10 Brandon Fajardo, 9 Thierry Lacrampe, 8 Brice Monzeglio, 7 Josefa Ulago Domolailai, 6 Loic Bernad, 5 Claude Dry, 4 Cameron Pierce, 3 Chris King, 2 Thomas Bianchin, 1 Julien Jacquot.
Replacements: 16 Quentin Lespiaucq Brettes, 17 Jérémy Hurou, 18 Euan Murray, 19 Giovanni Habel Kuffner, 20 Sean Dougall, 21 Samuel Marques, 22 Charly Malie, 23 Watisoni Votu.
Castres: 15 Geoffrey Palis, 14 Sitiveni Sivivatu, 13 Florian Vialelle, 12 Romain Cabannes, 11 David Smith, 10 Julien Dumora, 9 Julien Seron, 8 Johnnie Beattie, 7 Yannick Caballero, 6 Piula Faasalele, 5 Benjamin Desroche, 4 Christophe Samson, 3 Lucas Martinez, 2 Marc-Antoine Rallier, 1 Mihaita Lazar.
Replacements: 16 Julien Beziat, 17 Eric Sione, 18 Saimone Taumoepeau, 19 Mathieu Babillot, 20 Theo Hannoyer, 21 Francois Fontaine, 22 Remi Lamerat, 23 Antoine Dupont.
Referee: Ian Davies (Wales)
Assistant Referees: Wayne Davies (Wales), Martyn Lewis (Wales)