Grass samples from various field types in Denmark were dissected for stemboring fly larvae in order to estimate population densities. Oscinella species were predominant. The results support the assumption that O. vastator and O. pusilla are stationary species building up high population densities in perennial established fields, whereas O. frit is highly expansive primarily invading annual crops. From an O. frit aberrant larval type four species emerged: O. hortensis, O. nigerrima, O. pusilla and an O. frit resembling fly. O. hortensis was primarily found in stems of Festuca rubra and F. ovina, but also of Deschampsia flexuosa. Of the stemboring larvae 15.5% were parasitized by hymenopterous parasitoids, the wingless braconid Chasmodon apterus was responsible for 67,6% of the parasitism.