The systematic position of Amphidium has been controversial with the genus often treated either in the Orthotrichaceae near Zygodon or the Dicranaceae/Rhabdoweisiaceae near Rhabdoweisia. Determining whether Amphidium belongs to the Haplolepideae or Diplolepideae is difficult because the genus is eperistomate and its gametophyte shows weak morphological evidence supporting either position. Lewinsky (1976) placed Amphidium in the Orthotrichaceae because of its papillose calyptrae and similarities between the young capsules of Amphidium and Orthotrichum. The capsule evidence linking the genera rests on the presence of 16 trabecular filaments in the region of the spore sac. Haplolepideous mosses studied, in contrast, either lack trabecular filaments or have 16 bulging cells projecting into the air space (Kreulen 1972). Recent molecular evidence (Goffinet et al. 1998, Stech 1999, La Farge et al. 2000) indicates the genus belongs in the Haplolepideae. Within the Haplolepideae, Goffinet et al. (1998) suggested Amphidium may be close to Glyphomitrium (Ptychomitriaceae, Grimmiales), while Stech (1999) and Buck & Goffinet (2000) place the genus in the Rhabdoweisiaceae (Dicranales). Although LaFarge et al. (2000) considered their data were “insufficient” to indicate a family placement for Amphidium, they placed the genus, along with the Rhabdoweisiaceae, in the Pottiales.
In a recent revision Frahm et al. (2000) reduced Amphidium from 13 species to three and placed the genus in the Dicranaceae.