Aulacoseira granulata var. angustissima differs from A. granulata by having a combination of long and narrow valves with diameter of 2.5-4 μm and mantle height/valve diameter ratio greater than 3.2. Specimens with either valve diameter greater than 4 µm, or mantle height/valve diameter ratio lower than 3.2, or both, should be placed in A. granulata. Specimens of A. granulata var. angustissima with fine areolae can be confused with A. ambigua, which is recognizable by its hollow ringleiste and absence of long separation spines.