The Tigers failed to get out of their half in the opening 10 minutes and looked decidedly ring rusty as they trailed 6-0. But they soon moved into their stride and Steve Hackney ran in a hat-trick of tries.
The game was also memorable for debut tries for the Tigers' summer signings Rob Liley and Craig Joiner and from the young back row forwards Lewis Moody and Ben Pain.
Agen, skippered by the international Philippe Benetton, turned in a sparkling performance, but they were aided by a below-par performance from Cardiff who, although missing several first-choice players, were still able to field 10 full Welsh internationals. Cardiff's passing, tackling and general cohesion looked rusty.
Agen needed only four minutes to score when Stephane Longo touched down after an incisive break. Six minutes later Jannick Grassi scored under the posts and then Frederic Bourdeilh went clear wide on the left for a third try.
The Welsh launched a temporary revival when Jason Hewlett went over under the posts. The Agen hooker Brice Fabre burst through for his side's fourth try and then Abdelatif Benazzi achieved a pushover effort. Agen made it six before the first half was over. Cardiff salvaged some pride when the No 8 Owain Williams forced his way over, before the French completed the rout..