Gloucestershire 90 for 6 (Procter 3-13, Sanderson 3-22) trail Northamptonshire 327 (Rossington 94, Price 4-72) by 237 runs
Ben Sanderson and Luke Procter took three wickets each to put Northamptonshire firmly on top at Bristol as Gloucestershire were reduced to 90 for 6 replying to 327.
Northants captain Adam Rossington extended his overnight 66 to 94 - his highest score of the season - as the visitors collected three batting points.
Sanderson then took out the Glosters' top three before Procter's haul left the home side reeling.
Northants' total of 327 felt suspiciously over par on a pitch that offered something for the bowlers throughout. Gloucestershire couldn't make the most of it having won the toss and of their seven bowlers, only Ryan Higgins and Zafar Gohar went at under 3.2 runs an over.
Sanderson showed how it should be done in a typically skilful opening spell. He claimed wickets in his first, fourth and fifth overs to rip out the top order.
Ben Charlesworth pushed far too keenly at a ball from round the wicket that straightened a fraction and edged to second slip fourth ball of the innings. James Bracey got one that left him enough to take an edge to a good low catch by Emilio Gay at third slip and Miles Hammond fell in almost identical fashion.
It was then Procter's turn as he drew an edge from Tom Lace to first slip, got one to come back and beat Chris Dent's inside edge to take out off stump and lured Higgins into driving and edging behind.
It was an excellent day for Northants as Rossington resumed the fightback he began on the first evening to help his side built a very good total having been 147 for 5. He drove Matt Taylor on the up through cover, edged Jared Warner wide of second slip and pulled the same bowler imperiously wide of mid-on.
Higgins was lofted down the ground and Tom Price driven past mid-on. But he fell to the second new ball as Price got one to nip back between bat and pad.
Harry Goudlstone had fallen the same way for 43 in the previous over from Higgins, ending a sixth-wicket partnership of 124.
The dismissal began a smart reply by Gloucestershire with the second new ball as they took 5 for 56. Tom Taylor was the only lower-order batsmen to make an impression with 24 before he was last man out, driving at Warner and edging behind to a good diving catch from Bracey.
Northants vs Gloucs