Lewis Gregory recorded career-best figures of 6-47 as Somerset remained in control of their LV= County Championship Division One game at Northamptonshire.
The seamer helped the visitors dismiss Northants for 221, which gave them a deficit of 154.
Surprisingly, Marcus Trescothick did not enforce the follow-on and his decision was hardly supported by his and Nick Compton's dismissals, to David Willey and Muhammad Azhar Ullah respectively, 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 for three 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.
The total looked to be better than average and this was borne out by the lunch interval when the hosts had been reduced to 65 for four.
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 replacement wicketkeeper Alex Barrow with his second catch of the innings from a good Craig Overton delivery. Barrow was deputising for Craig Kieswetter, who was absent after being hit by a bouncer from Willey yesterday.
At 71 for five Northants were 155 short of the follow-on target but 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.
Thirty-eight short of making Somerset bat again, Willey went on the offensive and powered to a 42-ball fifty but his efforts should have been in vain when Gregory found Azhar's edge, only for James Hildreth to ground a straightforward chance at first slip.
However, Willey fell a couple of balls later as he drove at a wide Thomas delivery and the hosts would have been gearing themselves up to try and avoid innings-defeat number seven before Trescothick sprung his surprise.
Speaking about the decision to bat again, Somerset director of cricket Dave Nosworthy said: "I think it was an important decision to make in terms of, we'd only bowled 60 overs at them but we've come off another four-day game, we've had Twenty20 matches and we leave from here to go to another Twenty20 match.
"It's almost 10 days in a row we're on our feet so we need to be sensible how we manage it and there's a lot of time left in the game."
As for the home side's take on it and their chances in the contest, captain Middlebrook commented: "We weren't overly surprised as anything over 200 we thought they would bat again, even though we needed 225.
"We've got a chance if our seamers can front up in the first session. Who knows?"