Oxyurid nematodes Aspiculuris tetraptera and Syphacia obvelata are both common mouse intestinal parasites; in the same location several species of trichomonads occur. Tritrichomonas muris is the most often found, but there are also some others: Tritrichomonas minuta, Pentatrichomonas hominis or Hexamastix muris. It is known that, under some circumstances, trichomonads can be found in the intestine of mouse pinworms, as reported by Theiler and Farber (1936) for T. muris in A. tetraptera and S. obvelata. What is interesting, trichomonads were found also in pinworms from trichomonad-negative mice. In 1933, Becker observed motile trophozoites of T. muris in the egg of S. obvelata. These observations offer a chance that the protist may be transmitted to a new host in similar manner (via pinworm eggs) as it was described for Histomonas meleagridis. In our study we document trichomonads in pinworm bodies, determine species of trichomonads, search for their localization, morphology and, based on the data, assess possibility of trichomonad transmission to a new mammalian host via pinworm eggs. By use of classical histology, in situ hybridization and transmission electron microscopy, we observed trichomonads exclusively in the intestinal lumen of both pinworm species. They were found in the entire pinworm...