Cricket latest: Yorkshire in control thanks to Lyth
After winning the toss the visitors ran up 296-4, with Joe Sayers (50) and Anthony McGrath (73 not out) also enjoying the cloudless skies and flat pitch.
Opener Lyth looked untroubled for most of his 255-ball innings. The 22-year-old left-hander hit 15 fours in passing his previous best score of 132, made against Notts at Trent Bridge in 2008.
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Hide AdGary Ballance was the man who has been brought in for captain Andrew Gale, who is on England Lions duty against Bangladesh on Wednesday.
Yorkshire - who have won three and drawn two of their opening five County Championship games - named two spinners in Adil Rashid and David Wainwright.
Yorkshire won the toss and elected to bat.
Somerset: M E Trescothick (Capt), A V Suppiah, N R D Compton Z de Bruyn, J C Hildreth, J C Buttler (Wkt), P D Trego, A C Thomas M Kartik, B J Phillips, C M Willoughby.
Yorkshire: A Lyth, J J Sayers, A McGrath, J A Rudolph (Capt) G S Ballance, J M Bairstow (Wkt), A U Rashid, D J Wainwright T L Best, S A Patterson, O J Hannon-Dalby.