Species-level identification of male dolichopodid flies (Diptera: Dolichopodidae) is based on the structure of genitalia and secondary sexual characters. The effect of parasitic nematodes on the diagnostic features was studied in a sample of over 6000 Dolichopus adults collected mostly from Finland. Large nematodes were found in the abdominal cavity of 45 Dolichopus males. Parasitised males of the small to mid-sized species D. fraterculus Zetterstedt, D. lepidus Staeger, D. longitarsis Stannius and D. picipes Meigen were clearly demasculinised. Secondary sexual characters like a narrowed face, long ciliae on hind femora and modified tarsal segments were weakly developed or absent. Changes in the size, shape and position of male genitalia were also found. Larger species like D. discifer Stannius and D. popularis Wiedemann were less affected by the parasites. Some of the nematode-infected males have been described as new species and they are here synonymised as follows: Dolichopus brachyurus Zetterstedt 1859=D. fraterculus Zetterstedt, 1843; D. consimilis Wahlberg, 1850=D. picipes Meigen, 1824; D. cruralis Wahlberg, 1850=D. lepidus Staeger, 1842; D. kowarzianus Stackelberg, 1928=D. longitarsis Stannius, 1831; D. lapponicus Becker, 1917=D. lepidus Staeger, 1842; D. micropygus Wahlberg, 1850=D. fraterculus Zetterstedt, 1843; D. parvicaudatus Zetterstedt, 1843=D. plumipes (Scopoli, 1763); D. pectinitarsis Stenhammar, 1851=D. plumipes (Scopoli, 1763) syn. nov.