Northampton's 31-league game unbeaten run came ended when a Conor Wilkinson double helped Chesterfield to a 3-1 victory at the Proact Stadium.
The on-loan Bolton striker scored in each half and although Paul Anderson pulled a goal back, Dion Donohue sealed Chesterfield's first win in five games.
Chesterfield caught Northampton flat in the seventh minute when Jon Nolan played Wilkinson in on the left and his low shot went in off the far post.
Northampton almost conceded again in the 24th minute when Gboly Ariyibi's cross was deflected goalwards but Zander Diamond made a great clearance to put the ball over the crossbar.
It needed a good save from Adam Smith to deny Northampton in the 40th minute when Matt Taylor's 20-yard shot was turned over, but their hopes of a comeback were dented in the 49th minute when Wilkinson was allowed to turn on the edge of the area to score with a deflected shot.
Town reduced the deficit in the 63rd minute when Anderson was unmarked at the back post to beat Ryan Fulton, only for Dion Donohue to break away in the 70th minute and score with a 20-yard shot.
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