Thousands of people were taking a dip in the Parana River to cool off from the 100-degree heat Wednesday near the city of Rosario when people suddenly began complaining of bite marks on their hands and feet.
Officials blamed the attack on fish called palometas, a type of piranha with large, sharp teeth. The heat wave may have drawn the piranha to shore.
No one was killed but about 70 people were hurt. ABC News reports that one girl's finger was bitten off.
Following the incident, a paramedic said city beaches were closed, but it was so hot that within a half-hour, many people went back into the water.
According to ABC News, a piranha's bite is more powerful than a great white shark's and three times stronger than an alligator's. The carnivorous fish can also smell blood from up to 2 miles away.
The Associated Press and ABC News contributed to this report.