The diagnosis of Phanerothecium Kritsky & Thatcher, 1977 is amended, and Phanerothecioides n. g., Onychogyrodactylus n. g. and Aglaiogyrodactylus n. g., all comprised of oviparous gyrodactylids (Monogenoidea: Polyonchoinea), are proposed to accommodate 11 of the 15 species (14 new to science) herein described and/or reported from loricariid and pimelodid catfishes (Teleostei: Siluriformes) in Brazil: Phanerothecium harrisi Kritsky & Boeger, 1991, P. spinatoides n. sp. and P. deiropedeum n. sp. all from Hypostomus spp.; P. spinulatum n. sp. from Hypostomus cf. regani; Phanerothecioides agostinhoi n. g., n. sp. (type-species) from Hypostomus spp. and Pseudoplatystoma fasciatum; Onychogyrodactylus sudis n. g., n. sp. (type-species) and O. hydaticus n. sp. both from Ancistrus multispinus; Aglaiogyrodactylus forficulatus n. g., n. sp. (type-species) from Kronichthys lacerta; A. pedunculatus n. sp. from Hisonotus sp.; A. guttus n. sp. from Pseudotothyris obtusa; A. salebrosus n. sp., A. conei n. sp. and A. ctenistus n. sp. all from Pareiorhaphis parmula; and A. calamus n. sp. and A. forficuloides n. sp. both from Schizolecis guntheri. Phanerothecioides n. g. is characterised by oviparous forms lacking superficial and deep haptoral bars and pregermarial vitelline follicles, and by having a conspicuous testis in adult specimens, a syncytial prostatic gland, a reduced copulatory sac and vitelline ducts in the form of an inverted 'U'. Onychogyrodactylus n. g. is differentiated from all other oviparous gyrodactylid genera by its members having a spine-like accessory sclerite enclosed in a separate pouch associated with the terminal male genitalia. Species of Aglaiogyrodactylus n. g. possess H-shaped vitelline ducts and a complex accessory piece and sclerotised or non-sclerotised male copulatory organ enclosed within the copulatory sac.