Halicephalobus mephisto worms measure from 0.5 to 0.56 mm in length. Though species in the genus Halicephalobus have few distinguishing features, H. mephisto can be differentiated from other species within its genus by its comparatively long tail, which is between 110 and 130 micrometres in length. It is somewhat closely related to the mammalian pathogen Halicephalobus gingivalis, but is more closely related to certain unnamed species of the genus.[1]