Alex Hughes was the chief beneficiary of a rain-shortened day at the County Ground as he made his way to 74 not out, moving Derbyshire to 118 for four, replying to Northants' 255.
With only 29 overs possible in the morning session, Hughes moved from his overnight 38 to a fourth 50 in consecutive County Championship matches.
Under very gloomy skies with rain threatening, Hughes enjoyed a fair slice of luck with several plays and misses and an inside edge via the pad that just missed his leg stump.
But he seized on any chance to score and cut Ben Sanderson in front and behind point among his 10 boundaries and a six.
Northants did strike twice in the session, with Tom Lace, for 38, and Gary Wilson, for just three, edging and driving to Ben Curran at point.