Saracens produced an impressive second-half comeback to win the battle between the English and French league leaders in Castres.
Saracens trailed 9-3 at half-time at Stade Pierre Antoine before tries by replacements
Andy Saull and Schalk Brits sealed a vital win that keeps their hopes alive of qualifying from Pool Three.
Castres, the surprise leaders of the Top 14 in France, dominated the first half after taking an early lead through full-back Thomas Bouquie in the fifth minute.
Saracens equalised on 13 minutes through Glen Jackson's drop goal but spent the next ten minutes camped on their own half as Castres were twice held up over the try line.
Bouquie added two more penalties after Justin Melck was sin binned for a half-time lead but again Jackson levelled with two early kicks at the start of the second half.
Saracens were under pressure in the scrum but found success at the lineout from which the game's opening try came. A strong drive saw hooker Brits go close before Saull drove over from close range.
Steve Borthwick's side wrapped up victory nine minutes from time with their second try when Brits intercepted Alexandre Albouy's long pass to score under the posts.
Saracens moved to second in the pool, three points behind Toulon who recorded their biggest victory in Europe, beating Italians Rugby Rovigo 73-3.
Fijian wing Gabi Lovobalavu crossed four times, including the first and last of the French team's 11 tries at Stade Felix Mayol.
Fellow wing Sinoti Sinoti also scored twice while Felipe Contepomi added five conversions as Montpelier moved ahead of Worcester and Connacht in Pool Two with a comfortable 57-24 win over Olympus Rugby XV Madrid.
Full-back Thiery Benjamin scored 13 points, including of Montpelier's eight tries while Cesar Semperi bagged 14 points, more than Madrid had managed in their previous two outings in the pool.
