Reihana scored one try but had a nightmare off the boot
Northampton began their Heineken Cup campaign with a nervous, error-strewn victory at home to Castres.
The visitors broke the deadlock when Joe Tekori touched down before Bruce Reihana went over to level.
Cameron McIntyre's drop and five wide kicks - four from Reihana and one dismal Shane Geraghty effort - meant Saints trailed 8-5 at the break.
Saints led for the first time through a Ben Foden try before Geraghty and Stephen Myler kicks sealed the win.
Northampton enjoyed the bulk of the early possession but could not register their dominance on the scoreboard - Reihana missing the first of his kicks from just inside the Castres half after nine minutes.
Castres full-back Pierre Bernard sent a penalty wide from long range before Reihana missed off the tee once more.
Some well-orchestrated French pressure yielded points when Samoan lock Tekori burrowed over, requiring the approval of the Television Match Official.
Bernard failed with the conversion and the crowd were served up yet more dreary kicking when another Reihana penalty went askew straight in front of the posts and just outside the Castres 22.
New Zealander Reihana made amends somewhat for his boot calamities when he collected scrum-half Lee Dickson's pass to blindside before racing over unopposed.
On this occasion, Reihana's missed kick from the touchline for the extras was forgivable.
Castres regained the lead when McIntyre's drop sailed over, although the visitors had Ibrahim Diarra to thank for their three-point half-time advantage when a huge hit from the French flanker stopped a flying Geraghty in his tracks.
Mathieu Bonello was sin-binned for an intentional knock-down but Northampton could not make any more inroads before the break - Geraghty shinned another three-point attempt as the half-time whistle was greeted by an eerie silence at Franklin's Garden.
Just 35 seconds after the restart, Geraghty fed Phil Dowson whose neat inside offload to Foden allowed the England man to run over.
Geraghty converted to put four points between the sides, while Bernard squandered another kickable chance.
Saints were extremely fortunate when the referee ruled that Castres had knocked on as replacement prop Anton Peikrishvili crossed the line - replays suggesting Foden should have been penalised as he struggled to deal with the loose ball.
Bernard made it a one-point game with 20 minutes remaining, however three minutes later Geraghty restored his side's lead with a sweetly struck effort.
The 21-year-old Bernard came up short with another penalty but found his range with nine minutes left to set up a tense finale.
Myler's late strike came off the inside of the post, though, much to the relief of the home fans.
Northampton Saints director of rugby Jim Mallinder told BBC Radio Northampton:
"We were very nervous at the end. Castres played well, they got stuck into us and made life difficult.
"We missed kicks and we didn't put the pressure on the opposition. When that happens, it can end up being close.
"Both Shane Geraghty and Bruce Reihana had been kicking well, so it is frustrating."
Replacements: Ansbro for Foden (69), Myler for Geraghty (67), Murray for Mujati (67), Sorenson for Clark (74). Not Used: Sharman, Dreyer, Easter, Powell.
Castres: Bernard, Martial, Garcia, Cabannes, Inigo, McIntyre, Tillous-Borde, Forestier, Bonello, Saayman, Murray, J. Tekori, Diarra, Caballero, C. Masoe.
Replacements: Andreu for Martial (63), Sanchou for Garcia (78), Bai for Cabannes (48), Coetzee for Forestier (67), Kayser for Bonello (63), Rolland for Murray (69), Bornman for Diarra (71). Not Used: Peikrishvili.
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