Abstract Persistence of endophyte-free (E-) tall fescue ( Festuca arundinacea Schreb.) is erratic. Little information exists as to how fast endophyte ( Neotyphodium coenophialum )-infected (E+) tall fescue might encroach on E- tall fescue and whether specific conditions might influence the speed of encroachment. Plots of E+ and E- tall fescue genotypes 7 and 17 were established using a modified Nelder's design to compare performance of the E+ forms of the plants in pure and mixed communities at different population densities. The plots were planted at the USDA Southern Piedmont Conservation Research Laboratory in Watkinsville, Georgia, and the University of Georgia Plant Sciences Farm in Bogart, Georgia. Plants were grown over a 5 year period and dry matter yield monitored 1, 3, and 5 years after establishment. Relative crowding coefficients were calculated for each to establish trends of encroachment of the E+ on the E- plants in the mixed communities. Generally, dry matter yields of E+ tall fescue were greater than E- tall fescue regardless of whether they were grown in pure or mixed communities. As time progressed, the difference in dry matter yield between E+ and E- tall fescue grown in mixed communities was greater than that of the pure communities. Relative crowding coefficients increased as time progressed. Relative crowding coefficients at the Watkinsville location were greater after 5 years than those at the Plant Sciences Farm. Therefore, site specific conditions exist which affect the competitiveness of E- tall fescue and degree of encroachment by E+ tall fescue. Research is needed to identify which biotic, abiotic and management variables exacerbate encroachment of E+ tall fescue to better define the conditions which best suit E- tall fescue.