none Aiming to assess the mortality of the overwintering generation of Oscinella frit, populations of oscinellid flies and their parasitoids were sampled during five consecutive years, before and after overwintering. Samples were taken in permanent pastures, leys and ryegrass plots in Sweden. Densities of O. frit were higher in pastures than in leys. The percentage of oscinellids parasitized showed little variation among years, but was higher in ryegrass plots (37%) than in pastures (14%). The parasitoid complex was dominated by one species, Rhoptromeris heptoma. The percentage decline in density of unparasitized O. frit from autumn to spring ranged from 5 to 65% during the five studied winters. This mortality was not related to O. frit density in autumn, nor were any weather effects found. My results suggest that the yearly variation in mortality of overwintering O. frit is moderate and that, consequently, during most years it should be possible to predict spring population size based on population estimates made the previous autumn. Parasites diseases and disorders; Parasites; Insect parasites; Hosts; Insect hosts; Ecology; Population dynamics; Habitat; Terrestrial habitat; Land and freshwater zones; Palaearctic region; Europe Oscinella (Chloropidae); Hymenopteran parasites; Prevalence; Mortality; Overwintering mortality; Population density; Grassland; Sweden; South, winter mortality & hymenopteran parasites Oscinella frit (Chloropidae); Hymenopteran parasites; Prevalence; Mortality; Overwintering mortality; Population density; Grassland; Sweden Hymenoptera; Dipteran hosts; Oscinella; Prevalence; Grassland; Sweden; South; Prevalence in dipteran hosts Chasmodon apterus (Braconidae ); Ganaspis mundata; Hexacola hexatoma; Rhoptromeris heptoma (Cynipidae ); Halticoptera circulus; Trichomalus nanus (Pteromalidae ); Dipteran hosts; Oscinella; Prevalence, grassland; Sweden