Cover photograph (Copyright © 2000, American Society for Microbiology. All rights reserved.): Fusarium oxysporum growing on a low concentration of xylan, a complex mixture of plant cell wall polysaccharides. Congo red is used to stain the xylan deep red. Areas of partially degraded xylan appear pink. Scenarios similar to this, in which microorganisms are continuously confronted with suboptimal concentrations of structurally diverse nutrients, are prevalent in nature and probably serve as a common setting for microbial evolution. (For an article in this issue dealing with nonrandom mutation and microbial evolution under starvation and other stressful conditions, see page 2993.) (Photograph by A. Rojas and J. M. Anzola, Corpogen, Bogota, Colombia.)