Several forage species establish mutualistic associations with vertically-transmitted endophytic fungi that improve their fitness. However, the endophyte may be related to livestock toxicosis and hence ageing seeds has been used to remove the fungus since it loses viability before the seeds. The effect of endophyte on seed quality has rarely been studied. We conducted an experiment to study the relationship between the endophyte Neotyphodium occultons and seed germination characteristics in Lolium multiflorum. Infected and non-infected seeds were stored under six different conditions and seed germination rate and viability were monitored over two years. Seed water content was evaluated and the ageing-time model was used to describe the dynamics of seed germination in relation to storage period. The model provided a good description of the seed germination, and parameters appear to work better when seed viability is affected. The endophyte not only affected negatively the seed viability but also the germination rate. Although the endophyte modified the water content, this effect was not associated with the negative effect on viability. Since non-toxic endophytes are being selected to improve forage grasses, further research should incorporate the endophyte viability dynamics in the population-based model to better predict both seed quality decay and endophyte losses.