The finding of three immature nymphs of Prosopistoma Latreille, 1833 (Ephemeroptera, Prosopistomatidae) in an upstream site of Baishih River represents the first record of this rarely collected genus in Taiwan. These nymphs were discovered through extensive monthly sampling at the riffle habitats from 13 undisturbed sites over two years (Dec 2008–Nov 2010). The coloration pattern of the collected immature nymphs in Taiwan is similar to the immature stage of P. ocellatum and P. annamense, two species which have been found in similarly undisturbed, upland forested-stream habitats.