Bath triumphed in the battle of third and fourth in the Aviva Premiership, winning the physical battle and then killing off Harlequins' challenge with a stunning try by Jonathan Joseph.
While the back row of Francois Louw, Matt Garvey and Carl Fearns got through a ton of work to frustrate the visitors' attacking ambitions, Bath themselves had done little to threaten in the first half.
Instead they relied on three penalties from the boot of fly-half George Ford for a 9-3 half-time lead.
The try from Joseph, who has already trained with the England Elite squad, came out of the blue on 46 minutes when Harlequins were man down after Charlie Walker's sin-binning.
England's first choice full-back, Mike Brown, was left clutching thin air as the speedy Joseph ran round him to the line.
There was already a distinctly upbeat mood at a sold-out Recreation Ground as the Bath United paraded the A League trophy won earlier in the week.
The first team were seeking to extend their unbeaten record at home in the league to a full year and to post their 11th win on the bounce.
England prop Davey Wilson was the only absentee as Bath selected at near full-strength after runaway, back-to-back wins against Mogliano in the Amlin Cup. Anthony Perenise deputised at tighthead.
Harlequins, after seeing off Racing Metro in the Heineken, were without Danny Care at scrum-half and prop Will Collier so Karl Dickson and Paul Doran Jones stepped up.
Ford opened the scoring with a 30-metre penalty barely two minutes into the game after former Bath hooker Dave Ward failed to roll away from a ruck and was presented with an even easier chance to make it 6-0 shortly afterwards.
This time it was a mishit clearance by Nick Evans, snaffled by Ford, that forced the penalty from England skipper Chris Robshaw.
Quins might have scored at the other end after Dickson tapped a penalty close to the line, but Bath's defence held and forced a relieving penalty.
The visitors were exerting all the pressure and centre Tim Molenaar surged into the Bath 22 again, only to spill the ball in Ford's last-ditch tackle.
There was a fascinating contest at fly-half with Evans pulling the strings effectively for Harlequins while repelled their assaults with a succession of raking, rolling kicks.
And, when body checked by George Robson, the young playmaker put Bath 9-0 ahead from 40 metres on 25 minutes.
Bath's line-out work was far from perfect however and, following a line-out steal by lock Charlie Matthews, the home side conceded two penalties in quick succession. Evans finally put Quins on the board with a 30-metre kick in the 28th minute.
Just before the interval, Ford missed a couple of chances from difficult angles to restore Bath's two-score lead, but Quins did not get off scot-free as Walker was sin-binned for a tip-tackle on Watson.
Harlequins therefore started the second half a man short.
But that did not entirely explain how they allowed Joseph to pierce the defensive line so easily on 46 minutes. Ford's conversion was wide.
For once, Bath's line-out worked perfectly as Rob Webber found Hooper at the tail to provide rare, quick ball.
As the game opened up going into the final quarter, Bath's defence had to be just as alert to the threat of the Quins' backs but territory, possession - and the penalty count - were swinging the way of the home side.
Ford was short with a long-range kick on 65 minutes, however.
Robshaw rallied his players into a compelling spell of high-tempo rugby as the game drifted into the last 10 minutes but the Bath back row was equal to the task.
Harlequins were given a lifeline when Banahan was offside chasing Ford's cross-field kick, and they forced another penalty from loose play.
Replacement Tom Guest was held up on the line from the catch-and-drive though and Bath finished the job by marching back the visitors' scrum to force the penalty.
It did not matter that Ford, handed another penalty chance on the full-time whistle, hooked his kick.