Worcestershire collected two bowling bonus points but were held up by a fine century from Northamptonshire batsman Rob Keogh on day one of the Specsavers County Championship clash at New Road.
Ed Barnard (2), Joe Leach, Jack Shantry and Charlie Morris were amongst the wickets as the visitors were reduced to 172-5 mid-way through the afternoon session.
But then Keogh and Steven Crook added 159 for the sixth wicket in 38 overs.
The 24-year-old Keogh made 154, with three sixes and 21 fours from 236 balls, and Crook provided aggressive support with an unbeaten 84 from 123 deliveries in their side’s progress to 345-6 by the close.
Worcestershire skipper Daryl Mitchell put Northants into bat and the home side enjoyed two early successes.
Barnard, who shared the new ball with Leach, made the first breakthrough when Rob Newton (2) was caught off a leading edge by Charlie Morris making good ground from mid off.
It became 29-2 when Ben Duckett, having made a quickfire 26, drilled Leach straight to George Rhodes at cover.
Alex Wakely had a stroke of luck when he tried to pull Barnard when on 21 and the ball lobbed over the head of Rhodes at mid wicket.
But the visitors reached lunch without any further dismissals at 91-2.
The game swung in Worcestershire's favour during the early part of the afternoon session with three wickets falling.
In the second over after the resumption, Barnard enjoyed another success as Wakely (25) was caught behind by Ben Cox.
Then five overs later Mitchell held onto a low catch at second slip to send back Adam Rossington (7) off Shantry.
Morris got in on the wicket-taking act as Richard Levi (33) holed out to Joe Clarke at deep square leg at 172-5.
But then Keogh and Steven Crook swung the game towards Northants via a stand of 159 in 38 overs.
Keogh went to a fine century – his first in the Championship this summer – off 144 balls with two sixes and 15 fours.
Keogh was on 106 at tea and could have gone without addition when pulling Barnard to deep mid-wicket, but Tom Fell, having made ground to his left, was unable to take a difficult chance.
The end for Keogh, after nearly five hours, came from a misjudgement when he was run out by Barnard, swooping from mid-off as the batsmen went for a sharp single off Shantry.