Tall fescue [Lolium arundinaceum (Schreb.) Darbysh] infected with an endophyte [Neotyphodium coenophialum (Morgan-Jones & Gams.) Glenn, Bacon, & Hanlin comb. nov.] generally has agronomic advantages over endophyte free tall fescue.   Our main objective of this study was to determine if endophyte presence (E+) or absence (E-) in three tall fescue genotypes, affects concentrations of acid detergent fiber (ADF), neutral detergent fiber (NDF), nitrogen (N) and phosphorus (P) in field and greenhouse studies.  Field and greenhouse plants were harvested at three maturity stages: vegetative, early stem elongation (boot) and full bloom (mature).  E+ plants had higher concentrations of ADF and NDF (non-significant in one genotype) in the field study.  Endophyte lowered N concentration in the field from 19.2 to 18.5 g kg^-1.  Various interactions between genotype, endophyte, and maturity were significant for NDF, N, and P in both field and greenhouse studies.  This interaction occurred because the effect of the endophyte increased or decreased fiber, N, and P content of tall fescue depending upon plant genotype. This genotype by endophyte interaction should be kept in mind as novel endophytes are selected and placed into tall fescue cultivar selections, this interaction could alter fiber, N and P accumulation of the plants which depending upon plant genotype could result in a positive or negative response.