The attacks took place in the Parana River in Rosario some 300 kilometers (186 miles) northeast of Buenos Aires. Seventy people who were cooling off from high temperatures were also injured there in late December by the same piranha-like fish. They included seven children who lost parts of their fingers or toes.
The latest attack by the "palometas" was confirmed Saturday. They've been described by the local director of lifeguards as "a type of piranha, big, voracious and with sharp teeth that can really bite."
Media reports said the injured included a boy who suffered a foot wound while floating in the river.
Experts say unusually high temperatures during the Austral summer and lower numbers of species such as caiman that preyed on the fish could be causing the attacks.