A large region for exopolysaccharide (EPS) production that is involved in the pathogenicity of Erwinia amylovora strain CFBP1430, cloned previously, was further characterized in this study. An 8.4 Kb-long sub-cloned fragment from that ams (amy lovoran synthesis) region was mutagenized in vitro by Tn3Gus insertion. Several new chromosomal mutants were obtained by marker exchange. The phenotype of those mutants was determined with regard to pathogenicity, EPS production, and growth in planta. It appeared that alteration in EPS synthesis was correlated with altered pathogenicity and poor (or no) growth in planta. Altogether these data argue for a direct role of EPS in bacterial multiplication in planta, and/or for an indirect role by protecting the bacteria against host defence reactions.