A James Hildreth century and 94 from Pete Trego enabled Somerset to dominate proceedings for a second day against Yorkshire at Taunton where by close the champions trailed by 435 runs with nine first innings wickets remaining.
Hildreth and Trego added 199 for the fifth wicket in a Somerset total of 562 for seven declared before Trego was out in the nineties for the second time inside a week.
His dismissal was followed shortly by that of Hildreth for a superb 166, both falling to the leg-spin of Adil Rashid.
The Yorkshire reply began with a three-figure partnership between openers Adam Lyth and Alex Lees before Jamie Overton made a breakthrough for the hosts by having Lees caught in the slips by Trego for 33.
But Lyth remained unbeaten on 80 at the close, having faced 152 balls and hit nine fours and two sixes in a Yorkshire total of 127 for one.