A late try for Castres saw Glasgow Warriors record their fourth loss of the Heineken Cup this season.
Warriors looked the stronger side in the first half with some high-pace play, taking the lead just before half-time through a John Barclay try.
Castres could have gone in ahead, but Romain Teulet struggled with the kicking duties, missing two out of three penalties in the opening 40 minutes.
A scrappy start to second half saw few try scoring opportunities for either side.
Glasgow's discipline slipped towards the end of the match, as Tom Ryder and Byron McGuigan were both shown yellow cards, leaving the visitors with only 13 men on the pitch for several minutes, allowing Yannick Caballero to find the try line and put Castres ahead.
The late comeback denied Glasgow their first win in the Heineken Cup this season, while Castres move up to second in Pool 4.
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