Pseudomonas syringae pv. savastanoi causes yield loss by “olive-knot” disease in the olive breeding locations. Th ere are no suff icient information related to cellular and molecular mechanisms of disease development. Th e role of quorum-sensing in pathogenecity has been demonstrated in several plant pathogenic bacteria. In this work, Acyl-homoserine lactone (AHL) molecules responsible for quorum-sensing were investigated by using biosensor strains and HPLC in P. syringae pv. savastanoi isolates originated from Mediterranean region. As a result, synthesis of N-(3-oxo-hexanoyl)-L-homoserine lactone (OHHL) was demonstrated in bacterial cultures. P. syringae pv. savastanoi cells may use this molecule for sensing the population density which probably leads to the infection process.