Wesley Verhoek's first-half goal was enough to give ADO Den Haag back-to-back wins and condemn Ajax to just their second defeat of the season.
Verhoek's third goal of the campaign came a minute before the half-time whistle and allowed John van den Brom's side to build on last weekend's 1-0 success over Utrecht.
The loss means that Ajax, whose only other league defeat came at home to Utrecht last month, failed to move back level with leaders FC Twente who beat Excelsior yesterday.
Ajax started the game well and Mounir El Hamdaoui went close with a drive that just skipped wide, before Verhoek drew a flying save from home keeper Maarten Stekelenburg.
Luis Suarez made a mess of a chance that was presented to him six yards out midway through the first half, before El Hamdaoui failed with another attempt from just inside the area.
Ajax were quickly made to pay for their misses too, as Verhoek took in a pass from Nicolas Lodeiro and rammed home to give him side the lead.
Demy De Zeeuw did his best to get Martin Jol's men back into the game six minutes after the break but dragged a shot wide, while ADO remained a threat as Lex Immers took aim from just inside the area and forced Stekelenburg into a save.
ADO were looking the most likely to score at this stage and Verhoek nearly made it two when his right-footed hit from the edge of the box came back off a post with 28 minutes remaining.
Siem de Jong then found himself unmarked in ADO's box but was unable to cash in, and the visitors held on for the win.