Xylella fastidiosa is a xylem-inhabiting bacterium that causes Pierce's disease (PD) of grapevine. Growth rates of X. fastidiosa in a rich liquid medium were determined by culturing methods at various temperatures. The slope of the regression line between the points of 18 and 28 degreesC was similar to that reported for Escherichia coli between 12 and 30 degreesC and for Erwinia amylovora between 9 and 18 degreesC. For three PD strains, two almond strains, and an oleander strain, X. fastidiosa grew fastest at 28 degreesC but did not grow at 12 degreesC. Grape seedlings kept at 5, 10, 17, or 25 degreesC for 18 days, beginning 2 weeks postinoculation at 25 degreesC, had 230-fold lower populations of X. fastidiosa when kept at 5 degreesC, but populations did not change significantly over time at the other temperatures. In planta populations of X. fastidiosa decreased 3 days after placing the seedlings at 5 and 37 degreesC, and subsequent samples yielded no culturable bacteria at 37 degreesC. Based on in vitro and in planta studies, it appears that temperatures between 25 and 32 degreesC may be critical for the epidemiology of Pierce's disease because of its rapid growth rate at these temperatures, whereas temperatures below 12 to 17 degreesC and above 34 degreesC may affect the survival of X. fastidiosa in plants.