Three new species of gill monogeneans (Dactylogyridae: Ancyrocephalinae) are described from siluriform catfish from Iquitos, Peru: Demidospermus mortenthaleri n. sp. from Brachyplatystoma juruense (Boulenger), Demidospermus brevicirrus n. sp. from Pimelodus sp., and Aphanoblastella aurorae n. sp. from Goeldiella eques (Müller & Troschel). Demidospermus mortenthaleri is characterized by a male copulatory organ (MCO) with a small loop at its middle portion; 2 types of hooks, of which pairs 5 and 6 are longer than the remaining hooks; a proximal subunit round and highly depressed thumb; and a sclerotized vagina with a round pad at the vaginal aperture. Demidospermus brevicirrus is distinguished from other congeners by the presence of a short, straight, and robust MCO and boot-shaped accessory piece with a hooked projection directed posteriorly. Aphanoblastella aurorae is the only species of the genus that possesses an arrow-shaped sclerotized vagina and a medial process on the dorsal bar. Another 6 dactylogyrids described previously are recorded for the first time from the Peruvian Amazonia: Cosmetocleithrum bulbocirrus Kritsky, Thatcher and Boeger, 1986; Vancleaveus fungulus Kritsky, Thatcher and Boeger, 1986; V. janauacaensis Kritsky, Thatcher and Boeger, 1986; V. platyrhynchi Kritsky, Thatcher and Boeger, 1986; Unilatus unilatus Mizelle and Kritsky, 1967; and U. brittani Mizelle, Kritsky and Crane, 1968. Based on observations of specimens collected in the Peruvian Amazonia, new morphological data for these species are provided. Comparison of new specimens of U. unilatus and U. brittani with those of Unilatus brevispinus Suriano, 1985 and Unilatus longispinus Suriano, 1985, both originally described from Brazil, has shown that they are conspecific. Therefore, the latter species were synonymized with U. unilatus and U. brittani, respectively. In addition, 56 undescribed monogeneans found in catfish from the Peruvian Amazonia, some of them probably belonging to new genera, are listed.