Technical Abstract: Biotypes of Neotyphodium-tall fescue grass symbiota are provided with enhanced protection from grazing vertebrate herbivores and insects due to the production of toxic secondary metabolites. However, considerable controversy exists concerning the involvement of the Neotyphodium coenophialum and tall fescue symbiotum and toxicity to nematode species. A sterile in vitro system was developed to determine the interactive nature of known toxins specific to this mutualistic association and compounds known to be nematotoxic. The in vitro assay system developed used Pratylenchus scribneri as the target organism to determine the interactive nature of ergot alkaloids, the pyrrolizidine alkaloid (the lolines), and total phenolic fractions, and specific phenolic compounds. The in vitro assay is described along with methods for testing toxicity. The results indicate that while the ergot alkaloid is toxic to this species of nematode, there are possible potentiating effects observed when other alkaloids are mixed, suggestive of prior observations of toxicity on specific insects. Further, total phenolic and specific phenolic contents were determined in roots of endophyte infected and noninfected tall fescue, cultivar Jesup. HPLC analysis and UV mass spectrometry of root extracts revealed the presence of two major polyphenolics that were identified as chlorogenic and di-caffeoylquinic acids, both of which have known toxicity to several species of nematodes.