The champions are without pace bowler Steve Patterson, who will not play in the final two games as his father remains seriously ill.
Liam Plunkett comes into the side for Patterson, with leg-spinner Adil Rashid returning at the expense of Azeem Rafiq.
Yorkshire go into the game one point behind leaders Middlesex, who take on Lancashire at Old Trafford, and 21 points ahead of third-placed Somerset.
Yorkshire lost two early wickets in the morning session after winning the toss and choosing to bat.
Alex Lees, so prolific in last week’s win over Durham at headingley disappeared back to the pavilion without troubling the scorers – caught by Jim Allenby off Craig Overton.
Gary Ballance (3) had only added 18 with Adam Lyth before he followed Lees back to the locker room, caught by Lewis Gregory to give Overton his second victim.
Yorkshire: Lyth, Lees, Ballance, Gale (captain), Lehmann, Bresnan, Rashid, Hodd, Plunkett, Brooks, Sidebottom.
Somerset: Trescothick, Abell, Rogers (captain), Hildreth, Trego, Gregory, Davies, C. Overton, Leach, Groenewald, Allenby.