Low phosphorus availability alters the root systems in many plants, which may have adaptive significance in soil phosphorus acquisition. We investigated the effects of low phosphorus on the root system architecture of one cultivar (PDF=Pasture Demonstration Farm) of tall fescue (Lolium arundinaceum) either endophyte-free (E-), or infected with common toxic endophyte (Neotyphodium coenophialum) (CT E+), or novel endophyte genotypes AR542 E+ and AR584 E+ (Ag Research; Hamilton, New Zealand).  Combinations were grown in a growth chamber in phytagel nutrient salt plant culture media containing three KH₂PO₄ levels (0 mg/L, 0.31mg/L and 31 mg/L). Root system architectural parameters such as root length, root surface area, average diameter, root hair and lateral root densities were recorded using WinRhizo (Regent Instruments Inc., Ontario, CA) root image analysis software. Under low-phosphorus (0.31mg/L) and high-phosphorus (31mg/L) availability, endophyte-free (E-) had greater root length (156.7cm and 178.4cm, respectively), root surface area (8.97cm² and 10.67cm², respectively) and lateral roots densities, but lower phosphorus uptake when compared with common toxic endophyte CT E+, AR542 E+ and AR584 E+ at the same P concentration. More biomass (shoot and root dry weight) was produced and greater P uptake occurred in novel endophyte genotype AR584 E+ grown in high phosphorus media. Endophyte infection statuses did not affect root average diameter, regardless of the P availability levels. When P supply was adequate (31mg/L), plant biomass, root length, surface area and lateral root densities were similar among the four fungal endophyte-infection statuses but followed the general order E->CTE+>AR542 E+>AR584 E+, whereas P uptake showed the opposite tendency.  Results suggest that P uptake by fungal endophyte infected tall fescue, may not only rely on stimulating root growth, but also altering root system architecture and increasing root exudates secretion to acquire more nutrient for plants growth