Isospora parnaitatiaiensis Silva, Rodrigues, Lopes, Berto, Luz, Ferreira & Lopes, 2015 was identified from a new host, the plain antvireo Dysithamnus mentalis (Temminck), and also from the white-shouldered fire-eye Pyriglena leucoptera Vieillot, in its type-locality, the Itatiaia National Park in the southeastern Brazil, and a preliminary genotypic characterisation by sequencing the mitochondrial cytochrome c oxidase subunit 1 gene is provided. The oöcysts recovered from P. leucoptera and D. mentalis were polymorphic and have genotypic differences that were not considered sufficient for the description of new species, but only different genotypes and morphotypes of I. parnaitatiaiensis related to each host. These morphological and molecular variations were associated with a process of ongoing speciation and in adaptive development to their respective host species.