This large, beautiful riparian Fissidens is very rare but where it does grow, there can be a lot of it. Its shoots are typically 1-3 cm long and on a par with Fissidens dubius and F. adianthoides, both of which also have toothed leaf margins and several rows of pale cells along the leaf margin. Unfortunately, both species can also be found in the riverside habitat of F. serrulatus, so great care must be taken with identification.
In the field, F. serrulatus looks slightly glaucous and when its leaves are dry they have very little sheen. This is because the laminal cells are conically mamillose (a character best seen microscopically in a transverse section of leaf) and these scatter the light, making the surface look dull. Mamillae and papillae are similar but different! A mamilla is a bulge on the surface of a cell with a nipple-like tip whereas a papilla is a local thickening of the cell wall. The leaf cells of F. dubius and F. adianthoides have neither but do bulge slightly when viewed in TS. These species have shinier-looking leaves than F. serrulatus when dry.
F. serrulatus leaves usually have more teeth than the common species and these occur from leaf apex down to mid-leaf of below.