Plant pathogenic Pseudomonas syringae strains harbour a type III secretion pathway suggested to be involved in the delivery of effector proteins from the bacteria into plant cells. During plant interaction, the bacteria apparently produce surface appendages, termed Hrp pili, that are indispensable for the secretion process. We have created an insertion mutation library, as well as deletion mutations to hrpA, the structural gene encoding Hrp pilin. Analysis of the mutants revealed gene regions important for hrpA expression, pilus assembly and pilus-dependent autoagglutination of the bacteria. The majority of insertions in the amino-terminal half of the pilin were tolerated without bacterial interaction with plants being affected, while the carboxy-terminus appeared to be needed for pilus assembly. Insertions in the 5' non-translated region and the first codons within the open reading frame affected mRNA production or stability and abolished protein production.