Xylella fastidiosa, a pathogen of citrus, is the first plant pathogenic bacterium for which the complete genome sequence has been published. Inspection of the sequence reveals high relatedness to many genes of other pathogens, notably Xanthomonas campestris. Based on this, rye suggest that Xylella possesses certain easily testable properties that contribute to pathogenicity, We also present some general considerations for deriving information on pathogenicity from bacterial genomics.