Selenomonas ruminantium was originally placed with the Gram-positive Firmicutes phylum based on its 16S rRNA gene.

The observation of flagella under electron microscope revealed the presence of four rings, a typical sign of gram negative species.

Observation of the flagellar basal structure confirms that S. ruminantium has an outer membrane, a characteristic of Gram-negative bacteria .

The bacteria also exhibits polar or lateral flagellar structures depending upon the medium in which they are developed.

The shape and size of the hooks on the flagellae differ by the physiological conditions present, but the general structure is S-shaped.