The freshwater amphipod genus Hyalella in Brazil has the second highest diversity in South America; it is only surpassed by the diversity found in the Andean Lake Titicaca. Ten species of Hyalella are known for Brazil: H. gracilicornis (Faxon, 1876); H. longistila (Faxon, 1876); H. warmingi Stebbing, 1899; H. meinerti Stebbing, 1899; H. curvispina Shoemaker, 1942; H. brasiliensis Bousfield, 1996; H. caeca Pereira, 1989; H. montenegrinae Bond-Buckup and Araujo, 1998; H. pseudoazteca González and Watling, 2003; and H. dielaii Pereira, 2004. Two new species are described here, H. castroi and H. pleoacuta, both occurring in freshwater bodies of southern Brazil over 1.200 m high in the region of Aparados da Serra. A key for the species of Hyalella from Brazil is presented.