It is the third time that Castres have switched a match to Toulouse's home ground - which holds 19,000 as opposed to the 10,500 capacity of the Stade Pierre Antoine - with the first two coming last season against Munster (Heineken Cup) and Montpellier (Top 14) and follows Saints' own switch of venue to stadiumMK for its Heineken Cup pool match against Munster last season.
The switch has been made by Castres with the approval of tournament organisers ERC and the blessing of Saints whose chief executive Allan Robson believes that the switch will make travel easier for Saints supporters intending to travel to southern France.
"Castres is a nice town, but it is a difficult place to get to and the number and range of accommodation is quite limited," Robson said. "As we know from experience, Toulouse is considerably easier to travel to, has a wide variety of hotels to suit most budgets and can be accessed directly on flights from a number of UK airports.
"The away game against Castres will be very tough, and hopefully this switch may enable more Saints supporters to travel to France and get behind the team."
The Saints have also announced, with regret, that there will be no official club trip to the game.
"European away trips are a highlight of the season for many supporters and we looked at all possibilities as to how we could make it affordable for as many people as we could," Robson explained. "Unfortunately this just has not been possible. October is still in peak travel season and charter aircraft are in high demand, while scheduled airlines just don't have the space to accommodate the bulk bookings we would require.
"We also did not have a sufficient number of advance enquiries required to book our own charter aircraft, which is understandable given the current economic climate. Nevertheless we would like to thank those supporters who registered their interest, and hope that this news will not stop them from making their own arrangements to get to the game."