Lewis Gregory recorded career best figures of six for 47 as Somerset remained in control of their LV= County Championship game at Northants.
The seamer, who took the second five-wicket haul of the season, helped the visitors dismiss Northants for 221 which gave them a deficit of 154.
Surprisingly, Marcus Trescothick didn’t enforce the follow-on and his decision was hardly supported by his and Nick Compton's dismissals, to David Willey and Muhammad Azhar Ullah with the score in single figures.
Chris Jones, who took a while to get going, and Colin Ingram steadied the ship and although the latter fell to James Middlebrook in the final over of the day, a total of 72/3 gives Somerset a lead of 226.
Earlier in the day, Somerset’s first innings concluded on 375 when George Dockrell edged Andrew Hall to Adam Rossington behind the wicket.
This total looked to be better than average and this was borne out by the lunch interval when the hosts had been reduced to 65/4.
Gregory had got rid of James Middlebrook, Richard Levi and Rob Newton before Peter Trego, with the final ball before the break, forced Matt Spriegel to parry a short ball to third slip.
James Kettleborough, who had resisted gamely, became the fifth to fall when he provided sub wicketkeeper Alex Barrow with his second catch of the innings from a good Craig Overton delivery and at 71/5 Northants were 155 short of the follow-on target.
Adam Rossington, who survived a loud and prolonged appeal for a catch behind the wicket from Trego on 30, and Kyle Coetzer played their shots and more than doubled the total but neither looked like hanging around for too long and they fell to Alfonso Thomas and Gregory in quick succession.
Andrew Hall and Olly Stone became the fifth and sixth wickets for Gregory who should have had seven, two balls after dismissing the latter only to see Barrow spill a regulation chance offered by Azhar Ullah.
Thirty-eight short of making Somerset bat again, Willey went on the offensive and powered to a 42-ball 50 but his efforts should have been in vain when Gregory found Azhar Ullah's edge only for James Hildreth to ground a straightforward catch at first slip.
However, Willey fell a couple of balls later as he drove at a wide Thomas delivery and the hosts would've been gearing themselves up to try and avoid innings defeat number seven before Trescothick sprung his surprise.