The cestode Nomimoscolex sudobim Woodland, 1935 (Proteocephalidea) is redescribed on the basis of its type-specimens and recently collected material found in the intestine of the ‘sudobim’ Pseudoplatystoma fasciatum (Siluriformes: Pimelodidae) (type-host), from the Amazon River (near the type-locality). Nomimoscolex pertierrae n. sp. is described from the ‘surubi’ Pseudoplatystoma corruscans from the Paraná River around Porto Rico. Among fifteen known Nomimoscolex species, only N. sudobim possess a cortical uterus in combination with osmoregulatory canals some distance internal to the proglottis margins, as in N. pertierrae n. sp. N. pertierrae differs from N. sudobim, which exhibits the most similarity with the former species, in the morphology of the scolex, the size and unusual shape of microtriches (slender digitiform) on the internal and marginal regions of the suckers, and in the lower mean number of testes [(114 in N. pertierrae, ranging between 216 (recently collected material) and 240 (type-material) in N. sudobim)]. The material described as N. sudobim by Pavanelli & Rego (1992) from the intestine of P. corruscans is considered to be N. pertierrae. The utility of microtriches as characters for distinguishing between species is demonstrated in the clear differences in the shape of those of N. pertierrae and N. sudobim. The morphology of microtriches warrants more attention in future studies on the taxonomy of proteocephalidean cestodes.