The hrp gene locus of Pseudomonas syringae pv. pisi (P. s. pisi) race 1 was cloned in SuperCosl vector. The nucleotide sequences of hrpA, hrpB and hrpZ were determined, and the deduced amino acid sequences were compared with different races of P. s. pisi and corresponding genes in other bacteria. In particular, over 99% of the similarity in HrpZ (harpin) amino acid sequences was observed among the different races of P. s. pisi. Among P. syringae pathovars, very high similarity was observed, but the level of similarity was considerably lower than that of Erwinia. The HrpZ protein is rich in glycine but lacks cysteine. It is also heat tolerant as has been reported with other bacterial harpins. The carboxyl terminus of harpin also contains two directly repeated sequences of GGGLGTP and QTGT commonly found in P. syringae pv. syringae. It is very interesting to note that a tetrapeptide sequence, GDET, homologous to the hexapeptide Mycosphaerella pinodes-supprescin B (SSGDET) was found in HrpZ of P. s. pisi.