Higuchi & Iwatsuki (1994) considered Ectropothecium delicatulum a synonym of Isopterygium minutirameum; however, leaves of this species have a single inflated alar cell. Therefore, an Isopterygium species can have a single inflated alar cell at leaf base and small capsules, which shares similarities with Ectropothecium, but differs from the latter genus by filamentous pseudoparaphyllia and non-collenchymatous exothecial cells.