Rochdale slipped out of the play-off places after Robbie Simpson`s late winner.
Keith Hill's in-form side had been harbouring thoughts of a second successive promotion but striker Simpson brought them back down to earth with 11 minutes remaining.
Sam Saunders raced down the right-hand side and flighted in a teasing cross which Simpson met with a firm six-yard header.
Dale had earlier fashioned a series of chances which all went begging.
Will Atkinson fired wide from 12 yards and then skipper Gary Jones went close with a rasping 25-yard strike.
However, Brentford held firm and snatched victory through Simpson's predatory late strike.
Copyright (c) PA Sport 2009, All Rights Reserved.
Do YOU want to write for GiveMeSport? Get started today by signing-up and submitting an article HERE: http://gms.to/writeforgms