The extracellular polysaccharides produced as slime or capsule layers by bacterial pathogens of animals and plants have been often implicated as factors essential to pathogenesis. In the present study, virulence of the plant pathogen Erwinia amylovora was correlated with the ability to produce extracellular polysaccharide (EPS). EPS production by a series of field isolates and bacterio-phage-resistant mutants differing in the extent to which they cause symptoms in host tissues was examined by quantitation with a modified Laurell rocket immunoelectrophoresis assay. The amount of EPS produced as an easily removed capsular layer or slime on solid nutrient agar approximated the capacity to exhibit symptoms in host inoculation tests. Features common to the virulent isolates are mucoid colony morphology, sensitivity to EPS-specific bacteriophages [Sphi3 and PEal(h)], and ability to produce a characteristic EPS. Mutants selected for resistance to Sphi3 or nonmucoid colony morphology are deficient in EPS production and have lost the ability to multiply in host tissue and cause symptoms. We conclude that EPS may be directly involved in symptom expression and provide a function essential to the growth of the pathogen in host tissues.