Colony growth of Xylella fastidiosa (UCLA ID and STL PD strains) was quantified after incubation for 48 h in xylem fluid of Vitis rotundifolia Michx. cv. Noble and Vitis vinifera L. cv. Chardonnay. Xylem fluid was collected from grapevines in the field (dormant and growing season) and from container-grown plants in a screen house (growing season). Colony forming units(.)ml(-1) (cfu(.)ml(-1)) were counted 15 d after plating on periwinkle wilt (PW+) medium. Colony growth was promoted or inhibited compared to PW+ medium, and was dependent on X.fastidiosa strain, plant species and source of xylem fluid. The efficacy of cecropin A and B was tested against this bacterium. Colony growth of X.fastidiosa was greatly inhibited after a I-h-exposure to cecropin A or B. The minimum inhibitory concentration (MIC) of cecropin A or B for 100 % inhibition of X.fastidiosa was < 1 muM. The activity of cecropin B in xylem fluid of V rotundifolia cv. Noble was progressively reduced over time from 0.2 to 24 h. When 2 and 10 muM concentrations of cecropin A and cecropin B were mixed with xylem fluid for 24 h, a substantial amount of bacterial growth occurred after subsequent plating; shorter time intervals did not degrade the cecropins and did not prevent colony growth. Cecropin B (1 muM) added to xylem fluid of V rotundifolia cv. Noble and V vinifera cv. Chardormay for 24,48,72 and 96 h did not prevent subsequent colony growth. Colony number tended to be higher for V rotundifolia cv. Noble than V vinifera cv. Chardonnay. Tricine-sodium dodecyl sulphate polyacrylamide gel electrophoresis (Tricine-SDS-PAGE) of cecropin B in xylem fluid showed that cecropin B degraded completely (V vinifera cv. Chardormay) or almost completely (V rotundifolia cv. Noble) after 96 h.