The two-dimensional propagation of viruses through a &&lawn'' of receptive hosts, commonly called plaque growth, re#ects the dynamics of interactions between viruses and host cells. Here we treat the ampli"cation of viruses during plaque growth as a reaction}di!usion system, where interactions among the virus, uninfected host cells, and virus-producing host}virus complexes are accounted for using rates of viral adsorption to and desorption from the host}cell surface, rates of reproduction and release of progeny viruses by lysis of the host, and by the coupling of these reactions with di!usion of free virus within the agar support. Numerical solution of the system shows the development of a traveling wave of reproducing viruses, where the velocity of the wave is governed by the kinetic and di!usion parameters. The model has been applied to predict the propagation velocity of a bacteriophage plaque. Di!erent mechanisms may account for the dependence of this velocity on the host density during early stages of a growing plaque. The model provides a means to explore how changes in the virus}host interactions may be manifest in a growing plaque.