Epichloë fungal endophytes colonize the intercellular space of aerial organs of their plant hosts without causing symptoms. These symbionts are known to improve the performance of their host grasses in some situations of biotic and abiotic stress, leading to the suggestion that they can be used to improve grass tolerance to contaminants. The grass Festuca rubra is a host of the endophyte Epichloë festucae. We used two half-sib lines of F. rubra, each line composed of infected (E+) or endophyte-free (E−) seeds, to study the effect of varying levels of arsenic (6, 12, 25, and 50 mg L−1), and of the endophyte in seed germination and radicle growth. The results showed that seed germination was not significantly affected by arsenic (As) levels lower than 25 mg L−1, indicating that this grass has a relatively high tolerance of As at the germination stage. The decrease in germination observed at 25 and 50 mg L−1 was due to increased seed mortality and to the reversible inhibition of the germination of viable seeds caused by As. The presence of the endophyte did not change the germination response to arsenic of one line, but affected negatively the germination of the other line. In contrast to the process of germination, radicles of E+ seeds of both lines were longer than those of endophyte-free seeds. The results of this work indicate that Epichloë endophytes can affect the performance of some F. rubra genotypes when As is present in the soil.