In the past, large-scale aerial dispersal of microorganisms from sources such as evaporative cooling towers and sewage irrigation and treatment plants has been estimated by using atmospheric dispersion models based on temporal and spatial averaging as defined in the Gaussian plume models. The framework for a simulation model which describes the dispersion of individual droplets of water containing viable microbes is presented. The model accounts for physical, chemical, biological, and measured meteorological parameters of each droplet at each of many short steps. Repeating the modeling process for many droplets will simulate a cloud of droplets. The model is compared with the Tulelake, California release in 1988 and found to show very similar patterns of deposition within 30 m (the maximum observation distance of the source). A hypothesis for the survival sequence in the microbe-containing droplets is discussed. (Copyright (c) 1989, American Society for Microbiology.)