The endophytic fungi of the genus Neotyphodium live with their host grasses in a symbiotic relationship. Invisible from the outside, the fungi grow between the cells in the upper parts of the host plant. The host supplies the nutrients the fungus needs to survive and distributes the fungus through it’s seeds. For their part, the endophytes can improve the biotic and abiotic stress tolerance of their host plants. The improved biotic stress tolerance of the host plants is due to the endophyte-induced synthesis of various alkaloids, which are harmful for mammals and/or insects. Various investigations show the positive influence of the endophyte on the growth and the drought stress tolerance of tall fescue. On this behalf, the information about the symbiotic relationship between Neotyphodium lolii and Lolium perenne is rare and sometimes contradictory. This study investigates whether and how often, Neotyphodium-infected plants of commercially important grass species do occur on dry, flooded or alternatively both, dry and flooded sites in Saxony-Anhalt. To investigate the influence of endophyte-infection on the generative development and the growth of Lolium perenne genotypes, 14 Lolium perenne genotypes were examined in a single-plant field trial. Seven of these genotypes were tested in pot experiments for endophyte-effects on drought or alternatively flooding tolerance of the plants. Neotyphodium-infected grass ecotypes are widely distributed on natural grassland sites. Nearly all meadow and tall fescue sites did contain endophyte-infected plants, their mean incidence being usually above 75 %. The fine leafed fescues were infected only in some regions. On average, 51 % of the Festuca rubra and 26 % of the Festuca ovina ecotypes contained Neotyphodium endophytes. Neotyphodium lolii-infected Lolium perenne plants were found on 74 % of the tested sites, the mean incidence usually being lower than 50 %. For Lolium perenne and Festuca arundinacea it was verified, that the probability of occurrence of the endophyte-infected ecotypes on dry sites was significantly higher than on wet sites. The opposite was true for Festuca pratensis. The results of the single-plant experiments show that the effect of Neotyphodium lolii on the generative development and the plant growth of Lolium perenne genotypes can vary, depending on the specific endophyte-host-combination. The result of this variance can be negative or positive for the parameters measured. These effects can be caused by differences in compatibility between the symbiotic partners. Furthermore, the results of this experiment indicate, that there could be a relationship between the endophyte-effect and the plant productivity of a genotype. The fungus seems to improve growth of genotypes with low productivity, whereas growth of genotypes with high productivity remains unaffected or is even inhibited. This relationship should be verified in further experiments. A significant improvement of the drought stress tolerance by the endophyte was observed for two genotypes only. However, several genotypes showed an endophyte-induced increase in root growth. This effect can be of significance to plant survival especially at sites with stressful environment. Analogue with the endophyte-improved germination of seeds, it could explain the high incidence of endophyte-infected Lolium perenne plants on dry sites.