To study virus propagation, we have developed a method by which the propagation of the Lambda bacteriophage can be observed and quantified. This is done by creating a fusion protein of the caspid protein gpD and the enhanced yellow fluorescent protein (EYFP). We show that this fusion allows capsid formation and the modified viruses propagate on a surface covered with host bactera thus forming fluorescent plaques. The intensity of fluorescence in a growing plaque determines the distribution of phages. This provides a new tool; to study the propagation of infection at the microscopic level. Bioeng. 2007;96: 615-621. (c) 20066 Wiley Periodicals, Inc.