South African Thomas was signed as a Kolpak player

Alfonso Thomas's 5-46 put Division One leaders Somerset in contention for a victory against Yorkshire at Taunton.

The hosts had reached 258 all out in their first innings thanks to a useful 38-run final-wicket stand between Craig Kieswetter (67) and Charl Willoughby.

But Thomas tore into Yorkshire's middle order, dismissing them for 208 in their second innings, with only Richard Pyrah (51) and Andrew Gale (58) scoring runs.

Somerset were 12-0 at stumps on day three, 311 runs adrift of victory.

Somerset openers Marcus Trescothick and Neil Edwards were largely untroubled as they saw off the last five overs of the day and much could depend on Adil Rashid's leg spin on what promises to be a closely contested final day.

But the star on day three was seamer Thomas, the South African Kolpak signing, who finished with his best figures in county cricket.

Yorkshire began their second innings with a lead of 114 but were soon 0-2 as first-innings centurion Jacques Rudolph and Adam Lyth fell to Willoughby and Andrew Caddick respectively.

Anthony McGrath and Gale did a sound job taking the score to 76-2 at lunch, but in the second over after the interval Thomas found a gap between McGrath's bat and pad to bowl him for 21.

Gale followed lbw three runs later, having completed a solid half-century off 76 balls.

Thomas then bowled Gerard Brophy, Pyrah and Matthew Hoggard as his accurate swing bowling caused plenty of problems.