The aim of this work was to detect serologic evidence of influenza virus infections in South American fur seals (Arctocephalus australis) that inhabit the Uruguayan coast. In 29 of 37 serum samples that were analyzed, we identified antibodies to at least one of the following antigens: H1N1 (A/NewCaledonia/20/99), B/Beijing/184/93-like viruses, B/Hong Kong/330/01, and B/Sichuan/379/99 by means of the hemagglutination inhibition assay (HAI). Results confirmed that influenza A viruses circulate in marine mammals and also showed, for the first time, indirect evidence of influenza B infections in Arctocephalus australis.