Somerset closed day two at Taunton on 112-5 in their second innings after being forced to follow-on by Division One rivals Middlesex.
After being bowled out for 347 themselves, the visitors quickly seized control of the LV= County Championship clash by dismissing their opponents for just 152.
Tim Murtagh was the man who did much of the damage, claiming 4-28 first time around and then adding a further four victims when Somerset crashed to 35-5.
Peter Trego (58 not out) and Jos Buttler, who finished up unbeaten on 20, helped give the hosts some hope in the contest, though they still trail by 83 runs at the halfway stage.
Somerset's bad day began with a poor bowling effort, allowing the Middlesex tail to wag despite losing Neil Dexter for 82 to Jamie Overton, who finished with career-best figures of 6-95.
Murtagh (29) and Tom Smith (17 not out) put on 38 for the last wicket before the former then caused real damage with the ball, starting off by ending an opening stand of 36 by dismissing Marcus Trescothick for 24.
The next ball saw Alviro Petersen edge to wicketkeeper John Simpson and, with the Middlesex bowlers offering an example in consistent line and length, Somerset soon plummeted to 101-6.
Trego's defiant 34 delayed the inevitable, but Murtagh received good support from Toby Roland-Jones and Dexter, both claiming two wickets apiece, to give skipper Chris Rogers the chance to ask their rivals to have another go.
Somerset quickly found themselves in a huge hole; Arul Suppiah was pinned lbw for a duck by Murtagh, who then found a way through Petersen's defences to bowl him for four.
When Trescothick, on seven, was caught at point off a leading edge, the score was 11-3. It became 20-4 when Lewis Gregory went lbw and James Hildreth didn't last long either, edging Murtagh to first slip.