Xylella fastidiosa causes Pierce’s disease. a serious disease of grape, citrus variegated chlorosis. almond and oleander leaf scorches, and many other similar diseases. Although the complete genome sequences of several strains of this organism are now available, the function of most genes in this organism, especially those conferring virulence, is lacking. attachment of X. fastidiosa to xylem vessels and insect vectors may be required for virulence and transmission: therefore, we disrupted fimA and fimF, genes encoding the major fimbrial protein FimA and a homolog of the fimbrial adhesin MrkD, to determine their role in the attachment process. Disruption of the fimA and fimF genes in Temecula 1 and STL grape strains of X. fastidiosa was obtained by homologous recombination using plasmids pFAK and pFFK, respectively. These vectors contained a kanamycin resistance gene cloned into either the fimA or fimF genes of X. fastidiosa grape strains Temecula1 or STL. Efficiency of transformation was sufficiently high (approximate to600 transformants per mug of pFFK DNA) to enable selection of rare recombination events. Polymerase chain reaction and Southern blot analyses of the mutants indicated that a double crossover event had occurred exclusively within the fimA and fimF genes, replacing the chromosomal gene with the disrupted gene and abolishing production of the corresponding proteins, FimA or FimF Scanning electron microscopy revealed that fimbriae size and number, cell aggregation, and cell size were reduced for the FimA(-) or FimF mutants of X.fastidiosa when compared with the parental strain. FimA- or FimF(-) mutants of X. fastidiosa remained pathogenic to grapevines, with bacterial populations slightly reduced compared with those of the wild-type X.fastidiosa cells. These mutants maintained their resistance to kanamycin in planta for at least 6 months in the greenhouse.