Cover photograph (Copyright © 2004, American Society for Microbiology. All Rights Reserved.): Time series of photomicrographs showing gliding motility by Mycoplasma mobile on glass, annotated with computationally detected gliding paths. Images proceed from left to right and in successive rows at intervals of 1 s. The cells always glide in the direction of the tapered end by an unknown mechanism. The length of a single cell is about 1 μm. (See related article on page 4254.)