Technical Abstract: Purified cell-wall constituents or grape xylem sap added to media affected in vitro growth, biofilm formation, cell aggregation and gene expression of Xylella fastidiosa. Media containing xylem sap from Pierce¿s disease (PD)-susceptible plants provided better support for bacterial growth and biofilm formation than media supplemented with xylem sap from PD-resistant plants. Culturing X. fastidiosa on media containing various purified cell-wall constituents demonstrated that CM-cellulose, xylan, ß-D-glucan, k-carrageenan, cello-oligosaccharide and laminarin promoted bacterial growth whereas lichenan strongly suppressed growth. However, only laminarin, xylan, and k-carrageenan promoted biofilm formation in vitro. Lichenan, oligosaccharide, k-carrageenan, laminarin, xylan and ß-D-glucan all significantly decreased bacterial cell aggregation in vitro. Quantitative real-time PCR assays revealed that expression of genes encoding extracellular endoglucanase, endo-1,4-beta-glucanase, and periplasm protease were differentially regulated in response to amendment of media with xylem sap from PD- resistant and PD- susceptible grapevines. This study indicates that composition of xylem sap and cell walls influence basic parameters of X. fastidiosa biology which may, in turn, account for differences in pathogenesis of X. fastidiosa on PD-resistant and ¿susceptible grapevines.