Epichloë spp. are endophytic fungii of Pooid grasses that cause choke disease, the suppression of seed production. They also host Botanophila spp. the larvae of which feed on the fungus. Studies on E. elymi on wild grasses indicate that flies transfer spermatia between fungal mating types thereby affecting cross fertilization, suggesting that the fungus-fly interaction reflects obligatory mutualism. Epichloë typhina , inadvertently introduced into western United States, was first detected in cultivated orchard grass fields in 1996. It spread rapidly raising concerns about impacts on seed production. The present study was conducted to address questions pertaining to the occurrence and nature of the fungus-fly interaction in the new habitat of E. typhina. Surveys indicated no correlation between fly abundance and fungal fertilization. In one field, no fly eggs or larvae were detected but fertilized stromata were universally present. The fly was established in the remaining 12 fields surveyed but while number of stromata with fly larvae ranged from 6 % to 98 %, stromata development was uniform. Up to 10 larvae were present on a stroma, and these consumed > 90% of the perithecia. Comparison of pupal weights indicated that the fungal resource was not limiting even at high larval densities. An exclusion study in a D. glomerata field also indicated that E. typhina fertilization occurred without the fly. In Oregon, the fly clearly benefits from the association with the fungus but there is no evidence of benefit to the fungus.