Dear colleagues, A ps-file of the following short preprint can be picked up from my homepage (http://www.math.ruu.nl/people/moerdijk) I Moerdijk, J. Vermeulen, Proof of a conjecture of A. Pitts. Abstract: Using only elementary properties of inverse limits and localization, we prove the Beck-Chevalley condition for lax pullbacks of coherent toposes. In this way, we obtain a simple and constructive proof of the descent theorem for coherent (pre)toposes. With best regards, Ieke Moerdijk.