Technical Abstract: Mutations in the global regulatory genes gacS or gacA render Pseudomonas syringae pv. syringae strain B728a completely avirulent in foliar infiltration assays on bean. It had been previously demonstrate that gac genes regulate alginate production in Pseudomonas species, while other published work indicated a requirement for alginate in the pathogenic interaction of P. syringae on bean. Together, these results suggested that the effects of gacS and gacA mutations on virulence in B728a might stem directly from a role in regulating alginate. In this report, we confirm a role for gac genes in both algD expression and alginate production in B728a. We were unable, however, to demonstrate a role for alginate in pathogenicity, instead finding that B728a mutants completely devoid of detectable alginate were as virulent as wild-type in our assay. Thus, factors other than, or in addition to, a deficiency of alginate must be involved in the avirulence of gacS/gacA mutants.