A new species of Astyanax is described from the upper Rio Paraguai basin, Mato Grosso State, Brazil. The new species can be distinguished from congeners by having the body intensely yellowish in life (v. silvery, reddish or lightly yellow) and by morphometric and meristics traits. Astyanax dolinae n. sp. cannot be assigned to any of the Astyanax species complex currently recognized for the genus. It is only known from the Dolina Água Milagrosa, a karstic sinkhole lake, entirely fed by groundwater, surrounded by Cerrado, the savannah‐like vegetation of central South America.

A new species of Urocleidoides, U. astyanacis (Monogenea, Ancyrocephalinae) is described from the "lambari" Astyanax scabripinnis (Jenyns, 1842) and A. fasciatus (Cuvier, 1819). Hosts were collected in Atibaia and Jaguari Rivers near Campinas, São Paulo State, Brazil.<br>Uma nova espécie de Urocleidoides, U. astyanacis (Monogenea, Ancyrocephalinae) é descrita dos lambaris Astyanax scabripinnis (Jenyns, 1842) e de A. fasciatus (Cuvier, 1819). Os hospedeiros foram coletados nos rios Atibaia e Jaguari, próximo a Campinas, Estado de São Paulo, Brasil.