Résumé : The study of genetic regulatory networks has received a major impetus from the recent development of experimental techniques allowing the measuremen- t of spatiotemporal patterns of gene expression in a massively parallel way. This progress of experimental methods calls for the development of appropriate computer tools for the modeling and simulation of gene regulation processes. These tools should be able to deal with two major difficulties hampering modeling and simulation studies, viz. incomplete knowledge of the biochemical reaction mechanisms and the absence of quantitative informatio- n on kinetic parameters and molecular concentrations. We present a computer tool for the modeling and simulation of genetic regulatory networks, called Genetic Network Analyzer (GNA). The tool is based on a qualitative simulation method that employs coarse-grained models of regulatory networks. The use of GNA is illustrated in a study of the network of genes and interactions regulating the initiation of sporulation in Bacillus subtilis.