The pathogenicity of Erwinia amylovora is controlled in part by the type III protein secretion system, which is encoded by hrp genes. When grown in medium that induces hrp gene expression, E. amylovora strain Ea110 assembled pili. Immunogold labeling and genetic analysis revealed that the hrpA gene product is the main component of these pili (named Hrp pili). The hrpA gene is required for the secretion of two key virulence effector proteins, HrpN and DspA/E. In situ immunogold labeling showed that the secreted HrpN and DspA/E proteins were localized along the Hrp pilus, suggesting that the Hrp pilus functions as a conduit or gliding filament that guides the secretion of HrpN and DspE.