Xylella fastidiosa is a xylem-limited bacterial pathogen, and is the causative agent of Pierce's disease of grapevines and scorch diseases of many other plant species. The disease symptoms are putatively due to blocking of the transpiration stream by bacterial-induced biofilm formation and/or by the formation of plant-generated tylosis. Xylella fastidiosa has been classified as an obligate aerobe, which appears unusual given that dissolved O-2 levels in the xylem during the growing season are often hypoxic (20-60 mu mol L-1). We examined the growth and biofilm formation of three strains of X. fastidiosa under variable O-2 conditions (21, 2.1, 0.21 and 0 % O-2), in comparison to that of Pseudomonas syringae (obligate aerobe) and Erwinia carotovora (facultative anaerobe) under similar conditions. The growth of X. fastidiosa more closely resembled that of the facultative anaerobe, and not the obligate aerobe. Xanthomonas campestris, the closest genetic relative of X. fastidiosa, exhibited no growth in an N-2 environment, whereas X. fastidiosa was capable of growing in an N-2 environment in PW+, CHARDS, and XDM2-PR media. The magnitude of growth and biofilm formation in the N-2 (0 % O-2) treatment was dependent on the specific medium. Additional studies involving the metabolism of X. fastidiosa in response to low O-2 are warranted. Whether X. fastidiosa is classified as an obligate aerobe or a facultative anaerobe should be confirmed by gene activation and/ or the quantification of the metabolic profiles under hypoxic conditions.