We conducted a study to test the hypothesis that 'Kentucky 31' tall fescue with its fungal endophyte has agronomic performance typical of northern European tall fescues with their endophytes. The objective was to compare the growth of infected and uninfected Kentucky 31 tall fescue with infected and uninfected native tall fescue from populations from Finland and Sweden. Plant growth parameters and seed yield were measured in a field experiment with ten replicates of four main plots (fertility and irrigation treatments). In general we found endophyte-infected plants had higher plant weights than uninfected or endophyte-removed plants, especially in unfertilized treatments. Some Scandinavian tall fescue populations appear to be similar in agronomic performance to the widely-adapted cultivar Kentucky 31.