Cover photograph (Copyright © 2004, American Society for Microbiology. All Rights Reserved.): Blood agar plate containing a culture of Staphylococcus aureus DPC5246, a pathogenic strain which is sensitive to bacteriophage K (family Myoviridae). The bacteriophage, which has been traditionally termed polyvalent, infects a wide range of pathogenic staphylococci, including mastitic and methicillin-resistant strains. Interestingly, bacteriophage K was used as a therapeutic agent for different S. aureus infections in the former USSR. (See related article on page 2862.)