Neotyphodium lolii is a fungal endophyte of perennial ryegrass (Lolium perenne), improving grass fitness through production of bioactive alkaloids. Neotyphodium species can also affect growth and physiology of their host grasses (family Poaceae, sub-family Pooideae), but little is known about the mechanisms. This study examined the effect of N. lolii on net photosynthesis (P(n)) and growth rates in ryegrass genotypes differing in endophyte concentration in all leaf tissues.Plants from two ryegrass genotypes, Nui D and Nui UIV, infected with N. lolii (E+) differing approx. 2-fold in endophyte concentration or uninfected clones thereof (E-) were grown in a controlled environment. For each genotype x endophyte treatment, plant growth rates were assessed as tillering and leaf extension rates, and the light response of P(n), dark respiration and transpiration measured in leaves of young (30-45 d old) and old (>90 d old) plants with a single-chamber open infrared gas-exchange system.Neotyphodium lolii affected CO(2)-limited rates of P(n), which were approx. 17 % lower in E+ than E- plants (P 0.05). Neotyphodium lolii also decreased transpiration (P 0.05). E+ plants grew faster immediately after replanting (P 0.05). The endophyte effects on P(n) and growth tended to be more pronounced in Nui UIV, despite having a lower endophyte concentration than Nui D.Neotyphodium lolii affects CO(2) fixation, but not light interception and photochemistry of P(n). The impact of N. lolii on plant growth and photosynthesis is independent of endophyte concentration in the plant, suggesting that the endophyte mycelium is not simply an energy drain to the plant. However, the endophyte effects on P(n) and plant growth are strongly dependent on the plant growth phase.