Frustulia bahlsii is distinct from many other large Frustulia species by the blunt, spathulate ends of the longitudinal ribs; the porte-crayon is absent. Frustulia pangaeopsis also has blunt rib ends, but differs from F. bahlsii by shorter, narrower valves with a higher stria density. Frustulia bahlsii is larger and has a lower stria density than F. rhomboides.