This study was developed as a research of first order streams fish assemblages located in two Conservation Units in Northwestern Paraná, Brazil, with the purpose of providing information about ichthyofauna composition of these water bodies while they are still under protection. There were captured 2,557 individuals belonging to six orders, nine families and 10 species in three streams. The most abundant species were Phalloceros harpagos (44%) from Conceição stream, Astyanax aff. paranae (14%) and P. harpagos (12%) from Jurema stream, and Melanorivulus apiamici (9%) from Conceição stream. Trichomycterus sp. could not be identified into a specific level, meaning it can represent an undescribed species to science, registered for the first time at the upper Paraná River basin.