It is notoriously difficult to produce crystals of membrane proteins that diffract to sufficient resolution for structural studies by Xray crystallography. Crystals of a prokaryotic CLC chloride channel that were initially unacceptable for structural analysis improved in both quality and diffraction limit by a process of dehydration. The loss of water decreased the dimensions of the unit cell axes by up to 25 Angstrom, improved the diffraction limit from 8.0 to 4.0 Angstrom, and decreased the mosaicity to values of approximately 1degrees. Dehydration of integral membrane protein crystals should be one of the procedures included in the initial screening for appropriate crystals and as a method of improving the diffraction limits of existing crystals. (C) 2003 Elsevier Science (USA). All rights reserved.