A xylem-limited bacterium serologically related to strains of Xylella fastidiosa has been associated previously with citrus variegated chlorosis, a new and potentially serious disease of citrus in Brazil, When isolated and grown on PW (periwinkle wilt) medium, this gram-negative bacterium measured 0.4 X 4 mu m and was indistinguishable based on colony appearance from reference strains of X. fastidiosa obtained from the American Type Culture Collection, Rockville, MD. The bacterium also had a rippled cell wall typical of X. fastidiosa and induced symptoms typical of citrus variegated chlorosis in sweet orange after artificial inoculation. The bacterium was reisolated from petioles of symptomatic artificially inoculated plants, and its identity was confirmed by membrane entrapment immunofluorescence and Western blotting with antiserum UF-26 prepared against the original strain and extracts of petioles and midribs from inoculated plants. The organism was observed in large numbers in xylem vessels of diseased, but not healthy, plant petioles and in extracts of diseased, but not healthy, petioles, using a gold label with antiserum UF-26. The bacterium reisolated from symptomatic plant tissue was culturally, morphologically, and serologically indistinguishable from the strain used to inoculate the plants, completing Koch's postulates. Antiserum UF-26 reacted most strongly with strains of X. fastidiosa that cause diseases of grapevines, almond, ragweed, and oak. The citrus strain of X. fastidiosa also reacted as strongly as the homologous strain from plum to antiserum 1609-PP. This strain of X. fastidiosa may represent a new serological group intermediate between previously described serogroups of X. fastidiosa.