Relegation-threatened Somerset made in-roads into Lancashire's top order to apply pressure on the second day of the Specsavers County Championship First Division match at Taunton.
Replying to the home side's first-innings 335, the red rose county lurched to 39-3, losing Alex Davies, Haseeb Hameed and Liam Livingstone in quick succession.
Veteran campaigners Shivnarine Chanderpaul and Steven Croft steadied the ship thereafter, but the visitors are up against it after reaching lunch on 61-3, still 274 runs behind..
Intent upon stamping their authority, Somerset effected the desired early breakthrough, the impressive Craig Overton locating Davies' outside edge and Marcus Trescothick pouching a catch of the regulation variety at second slip.
Having batted for more than an hour and faced 56 balls in accruing four runs, England batsman Hameed was lured into uncharacteristic indiscretion by a short-pitched delivery from Trego and top-edged a hook to George Bartlett, running in from fine leg.
Introduced as early as the 13th over, slow left armer Jack Leach claimed his 150th first-class victim when Livingstone, pushing forward tentatively, edged low to first slip where Overton stooped to conquer.
Resuming their first innings on 330-9, Somerset's tail-end resistance spanned a mere seven balls, the home side adding a further five runs before Leach edged Tom Bailey to Dane Vilas in the slip cordon.