Warwickshire, third in the table, were dismissed for 276, two behind Somerset, and at the close their hosts were 146 ahead with seven wickets standing, thanks largely to Mark Lathwell's 10th half-century of the season.
Chris Cairns led Nottinghamshire's recovery against Gloucestershire at Worksop with an unbeaten 107, which included three sixes and 17 fours. The dashing New Zealander added 147 for the sixth wicket with Paul Johnson, who fell just two short of his third century in successive matches.
Surrey's Darren Bicknell hit his first Championship hundred of the season, an unbeaten 120, to frustrate Lancashire at Lytham where only five wickets have fallen for 629 runs in two full days.
Graeme Hick gave England a taste of what they have discarded with three stunning slip catches and two wickets for Worcestershire against Hampshire, but Malcolm Marshall and Jonathan Ayling steered the visitors to safety.
A polished 69 by Jason Pooley killed Kent's hopes of forcing Middlesex to follow-on at Canterbury.
Steve Watkin, the Glamorgan seamer, and Ian Salisbury, Sussex's leg-spinner, both took their 50th wicket of the season at Eastbourne.Reuse content