Restriction fragment length polymorphism (RFLP) analysis was used to determine similarities and differences in DNAs among 24 strains of Xylella fastidosa from eight different hosts. Two genomic libraries were constructed separately in pUC18 from two strains of X. fastidiosa: strain R112V2, isolated from grapevine showing Pierce's disease symptoms in Georgia; and strain PLMG83 (ATCC 35871), originally isolated from plum with leaf scald disease. The 12 probes from the R112V2 library, together with two restriction enzymes (24 probe-enzyme combinations), generated 67 scorable characters. Similarity coefficients (S(xy)) among the 24 X. fastidiosa strains ranged from 0.58 to 1.0. Sixteen strains from Pierce's disease of grapevine examined had striking genetic uniformity (S(xy) > 0.87), suggesting the presence of Pierce's disease RFLP group in X. fastidiosa. RFLP analysis, using probes from the PLMG83 library, yielded similar results.