Magellanic penguins swim after being released in the Atlantic Ocean
The Center of Coastal and Marine Studies (Ceclimar) said veterinarians were investigating the deaths of the 512 marine animals which beached on the coast between the towns of Tramandai and Cidreira, some 100 kilometres (60 miles) from the state capital, Porto Alegre.
Some 30 samples from the penguins were being analyzed at Porto Alegre University and results were to be released within a month, it added.
Ceclimar officials told Globo's G1 website that veterinarians were puzzled by the large quantity of animals found and by the fact that they appeared well fed, not exhausted and without injuries or oil stains.
These Magellenic penguins, named after the Magallenes region in which they breed, mate in large colonies in southern Argentina and Chile.
They traditionally migrate north between March and September along the Rio Grande do Sul coast to head up to Sao Paulo.
Their diet consists mainly of small fish and marine crustaceans and their chief enemy is the southern sea lion.