A model of the growth of bacteria on leaf surfaces was developed using a combined continuous and discrete simulation ap proach. It is important to understand the population dynamics of these bacteria under field conditions because of their role in causing disease and frost damage to plants, and their potential as biological control agents and enhancers of precipitation. The model incorporates many of the necessary field variables, and is designed to be part of a larger ecological model involving arrivals and departures, inimical organisms, and involvement in atmospheric processes. Historical validation indicates a reasonable correlation between predicted and observed population levels in the field. Analysis of simulation results suggests an explanation for the observed lognormal distribution of bacteria in the simuland.