Maurice Chambers claimed three wickets as Northamptonshire secured a first-innings lead over Sussex in the Second XI Championship at Finedon.
Chambers picked up 3-39 - with a couple of successes for left-arm spinner Saif Zaib - to help dismiss the visitors for 215 in reply to the County's 303 all out.
Willian Vanderspar held Sussex together with 78, but Northamptonshire began their second knock 88 runs to the good.
They closed on 80-4 - an overall lead of 168 - with Lloyd Paternott hitting 37.
The match finishes on Thursday.