Cover photograph (Copyright © 2004, American Society for Microbiology. All Rights Reserved.): Scanning electron micrograph showing aerial hyphae in the form of coils on the surface of a colony of Streptomyces coelicolor. This filamentous bacterium undergoes an intricate process of morphological differentiation that involves the conversion of aerial hyphae into chains of spores. (Photo by Joel Schmid.) (See related article on page 3570.)