Bath's Investec Champions Cup hopes suffered an immediate setback as twice-winners La Rochelle triumphed 24-20 at the Recreation Ground.
The Gallagher Premiership leaders launched a powerful second-half fightback, scoring two converted tries from 15 points adrift, but La Rochelle had just enough in the tank for a priceless away win.
They roared clear through tries from flanker Oscar Jegou, prop Reda Wardi and scrum-half Tawera Kerr-Barlow, while fly-half Ihaia West kicked three conversions and a later penalty.
Bath were in deep trouble at half-time, managing only two Finn Russell penalties, yet they stirred after the break as hooker Tom Dunn and lock Quinn Roux claimed touchdowns, with Russell adding the extras.
Despite swirling rain and a gusting wind, a compelling contest unfolded, although it ramps up pressure on Bath, despite a losing bonus point, to potentially win at least two of their last three pool games as they chase a last-16 place.
They tackle Benetton in Italy next, with Clermont Auvergne and Leinster looming as testing opponents in January.
Gloucester 15-10 Edinburgh
Gloucester defied tough conditions to get their European Challenge Cup campaign off to a successful start with a hard-fought 15-10 win over Edinburgh at Kingsholm.
A second-half try from Albert Tuisue proved the difference in a tight encounter as the hosts took advantage of sin-binnings for Edinburgh's Tom Dodd and Marshall Sykes.
The score had been locked at 7-7 at the interval after Seb Blake's early touchdown was cancelled out by Wes Goosen.
The hosts' other points came with a conversion and penalty from Santiago Carreras. Edinburgh, who at least left with a bonus point, also got on the scoreboard with a Ross Thompson conversion and Ben Healy penalty.
Dragons 14-18 Montpellier
Dragons were edged out 18-14 by Montpellier in their opener in the face of strong winds at Rodney Parade.
All the points were scored in the first half with the Welsh side battling back from an 18-point deficit, as tries from Madosh Tambwe and Marco Tauleigne - after two Aurelien Barreau penalties - got the visitors off to a flying start.
Dragons fought back with tries from Taine Basham and Angus O'Brien, who converted both, but were unable to maintain the momentum after the break.
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