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°C was similar to that reported for Escherichia coli between 12 and 30°C and for Erwinia amylovora between 9 and 18°C. For three PD strains, two almond strains, and an oleander strain, X. fastidiosa grew fastest at 28°C but did not grow at 12°C. Grape seedlings kept at 5, 10, 17, or 25°C for 18 days, beginning 2 weeks postinoculation at 25°C, had 230-fold lower populations of X. fastidiosa when kept at 5°C, 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°C, and subsequent samples yielded no culturable bacteria at 37°C. Based on in vitro and in planta studies, it appears that temperatures between 25 and 32°C 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°C and above 34°C may affect the survival of X. fastidiosa in plants.