Derby County 0 Grimsby Town 1
A first-half goal from Gary Jones was enough to give Grimsby a well earned victory and condemn Derby to another cup night they will want to forget.
Derby manager Phil Brown made eight changes and it showed, in a disjointed display that allowed Grimsby to dominate much of the first half.
The League Two side could have scored more goals and it was only in the closing stages that Derby finally put the visitors under any kind of pressure.
There were anxious moments as Derby forced two corners in stoppage time, but Grimsby held on to give their 600 plus fans a victory to celebrate.
Derby paid the price for poor defending when Jones was allowed to take a throw-in on his chest and turn to fire a shot from ten yards past Lee Grant.
Derby responded with a breakaway that saw Tommy Smith shoot just wide, but Grimsby almost scored another when Gary Cohen put a cross from Paul Bolland the wrong side of Grant's left-hand post.
Grimsby continued to have the best of the midfield in the second half, and they went close again when Michael Reddy headed over from ten yards.
Smith was Derby's main threat, and he flashed a shot wide on the hour before Paul Peschisolido had Derby's first effort on target 5 minutes later with a 20 yard shot straight at keeper Steve Mildenhall.
But Grimsby were doing more than just hanging on, and the impressive Jean-Paul Kalala Kamudimba sent Reddy away on the right, but his shot was charged down when he turned inside.
Gary Jones set up Bolland in the 69th minute, but Grant saved well to keep Derby in the game and Steve Mildenhall made his first real save when he dived to his left to deny Adam Bolder.
Derby's best chance came in the 76th minute when Smith picked out Grzegorz Rasiak, but the Pole headed high over the bar from six yards, and although the Championship side pressed hard in the final minutes, Town kept them at bay to earn a place in the second round.