In this paper we describe a new Monogenetic Trematode of the genus Loimos MacCallum, 1917 (Loimolinae Price, 1936; Loimoidae Bychowsky, 1957) from the gills of Scolidoni serraenovae (Rich), and we give a new record for Tagia ecuadori (Meserve, 1938) Sproston, 1946, at the south Atlantic Coast, with a brief description. Loimos scitulus sp. n. differs from the other species of the genus by the form of the proaptor, by the big ootipe, by the number of testicles, by the position of the genital pore, by the filament of the egg and by the form of the opistaptor. There are simmiliarities with the proaptor of Loimos salpinggoides; with the position of the genital pore of Loimos secundus; and with the opistaptor of Loimos winteri. Although the specimen of Tagia ecuadori were found to have a muscular vagina, we identified them to this species for all the structures and measures are in the variation given for this group.