La Rochelle down Quins for play-off spot
It had looked as though their passage to the last eight was going to be comfortable as they opened up a 13-0 interval lead with tries from Pierre Aguillon and Kini Murimurivalu, plus a conversion and penalty from Alexis Bales.
But Harlequins, who had rallied from 21-0 down to beat Wasps 33-28 at home last weekend for their first win of this season's competition, had other ideas.
They fought back strongly and grabbed a try through England scrumhalf Danny Care converted by Marcus Smith to leave La Rochelle hanging on grimly in worsening conditions.
The French side's win, combined with Wasps' bonus-point home victory over Ulster Rugby in the other Pool 1 game, ensured they leapfrogged over the Irish province to head the Pool.
The scorers:
For La Rochelle:
Tries: Aguillon, Murimurivalu
Pens: Bales 2
For Harlequins:
Try: Care
Con: Smith
La Rochelle: 15 Kini Murimurivalu, 14 Rene Ranger, 13 Paul Jordaan, 12 Pierre Aguillon, 11 Vincent Rattez, 10 Jérémy Sinzelle, 9 Alexi Bales, 8 Victor Vito, 7 Afaesetiti Amosa, 6 Levani Botia, 5 Mathieu Tanguy, 4 Jason Eaton (captain), 3 Uini Atonio, 2 Pierre Bourgarit, 1 Dany Priso
Replacements: 16 Jean-Charles Orioli, 17 Lekso Kaulashvili, 18 Mohamed Boughanmi, 19 William Demotte, 20 Romain Sazy, 21 Tawera Kerr-Barlow, 22 Benjamin Noble, 23 Eliott Roudil
Harlequins: 15 Ian Prior, 14 Ross Chisholm, 13 Alofa Alofa, 12 James Lang, 11 Tim Visser, 10 Marcus Smith, 9 Danny Care, 8 Mat Luamanu, 7 Archie White, 6 James Chisholm, 5 Ben Glynn, 4 George Merrick, 3 Phil Swainston, 2 Dave Ward (captain), 1 Lewis Boyce
Replacements: 16 Elia Elia, 17 Mark Lambert, 18 Josh McNulty, 19 Charlie Matthews, 20 Dino Lamb, 21 Dave Lewis, 22 Joe Marchant, 23 Aaron Morris
Referee: George Clancy (Ireland)
Assistant referees: Frank Murphy (Ireland), Leo Colgan (Ireland)
Television match official: Brian MacNeice (Ireland)