Steve Magoffin produced a sensational opening burst to blow Northamptonshire’s top order away on the second day at Hove.
The Australian seamer dismissed Stephen Peters, James Kettleborough and Matt Spriegel for ducks in his first two overs to leave the visitors in deep trouble on 1-3 at lunch.
That was in reply to Sussex’s first innings total which was boosted from 319-6 overnight to 405 all out with Luke Wright making 158.
Magoffin struck with his third ball as he beat the defences of Peters and two balls later Kettleborough was following him back to the pavilion having edged to James Tredwell at second slip.
Spriegel lasted seven balls before Magoffin knocked over his off stump to leave Northants on 0-3 after three overs.
The visitors could have suffered further damage as James Middlebrook and Rob Newton scampered a single to get off the mark where a direct hit would have resulted in a run out.
Earlier, Wright and Tredwell had taken their partnership to 53 and earned Sussex a fourth batting point before Tredwell was bowled for 31 by Steven Crook.
Wright passed 150 for the second time this season and the fourth time in the last two years before finally miscuing a catch to the man running in from long-on.