Harlequins (14) 19
Tries: Monye 2 Pens: Evans 3
Toulouse (0) 23
Tries: Donguy, Sowerby Cons: Elissalde 2 Pens: Elissalde, Michalak, Fritz
Toulouse showed all their European know-how to defeat Quins
Harlequins were beaten by European giants Toulouse in a thrilling encounter at the Stoop which puts a big dent in their Heineken Cup hopes.
The home side dominated the first half as the French team struggled to settle and three Nicks Evans penalties and Ugo Monye's try put them 14-0 ahead.
Toulouse levelled with tries from Yves Donguy and Shaun Sowerby but Monye put Quins in front again with another try.
However, Toulouse edged home with three penalties in the final 21 minutes.
The result is a huge blow to Harlequins, who are now adrift at the bottom of pool five which leaves them with an uphill battle to qualify for the knockout stages.
Toulouse, three-time winners of the Heineken Cup, were dreadful in the first half, making a catalogue of errors and failing to put Quins under any kind of pressure.
Quins, last year's quarter-finalists, were 6-0 ahead after 22 minutes following a truncated opening which saw three players - including skipper Will Skinner - go off injured.
The home side scored the first try on 28 minutes, England wing Monye coming into the midfield and producing an electric outside break to touch down.
Toulouse were on the ropes and after a break from wing David Strettle, they were penalised for slowing the ball down and Evans made the score 14-0.
The visitors started to regain some composure just before the break but in a passage of play that summed up their half, they spilled the ball as they were beginning to threaten.
Following what was bound to have been a half-time dressing down from coach Guy Noves, Toulouse were a completely different proposition in the second half and two quick-fire tries levelled the match at 14-14.
The first came when a kick from the out-of-sorts Maxime Medard was allowed to bounce, creating havoc in the Quins defence which gave Yves Donguy a simple run to the line.
The second try came three minutes later after Quins had to scramble back when they failed to deal with an Elissalde high ball. From the resulting line-out, number eight Sowerby came off the back of a maul to crash over.
Quins did not take long to respond and they took the lead again when Monye was the first to react to a well-judged grubber kick from Mike Brown to the corner.
With their side 19-14 ahead, the vocal home support were thinking of another memorable win over French opposition but Toulouse had other ideas and their European know-how came to the fore as they began to grind Harlequins down.
Jean-Baptiste Elissalde reduced the deficit to two points when Quins were penalised for failing to release on the floor while replacement Frederic Michalak put Toulouse ahead for the first time with another penalty on 63 minutes.
Quins should have sealed the victory but Brown and Tom Guest failed to turn two excellent scoring opportunities into points by failing to pass the ball with Toulouse scrambling.
Michalak missed a kickable penalty and then hit the bar with a long-range drop-goal attempt but Florian Fritz made sure with a 40-metre penalty 90 seconds from time.
Harlequins head coach John Kingston:
"I thought we were outstanding in the first half and the pressure we applied dictated how they played. Few teams around would have lived with us in that period.
"But we didn't apply the same pressure early in the second half, plus it was freakish the way the first try came about with a bouncing ball, and it gave them fresh heart.
"Nobody would have complained if Harlequins had won that game but there is a very, very narrow line between success and failure at this level.
"That was far and away the finest rugby we've played this season, so we are frustrated. There's no question our situation in the Heineken Cup now is very difficult, but if we can do the business home and away to Sale, beat Cardiff Blues here, then who can tell what will happen with the other results."
Harlequins: Brown; Strettle, Tiesi, Masson, Monye; N Evans, Care; Jones, Fuga, Lambert, Stevenson, J Evans, Robshaw, Skinner, Easter.
Replacements: Clegg for Brown (77), Lowe for Monye (77), Brooker for Fuga (79), Andress for Lambert (65), Guest for Skinner (10).
Not Used: Croall, O'Connor, So'oialo.
Toulouse: Medard, Clerc, David, Jauzion, Donguy, Elissalde, Kelleher, Poux, Servat, Lecouls, Millo-Chluski, Albacete, Nyanga, Dustautoir, Sowerby.
Replacements: Fritz for David (55), Michalak for Elissalde (62), Human for Poux (3), Johnston for Lecouls (55), Lamboley for Millo-Chluski (68), Bouilhou for Nyanga (12), Picamoles for Sowerby (55).
Not Used: Basualdo.
Ref: Alain Rolland (Ireland).