Limited information exists on avian influenza (AI) virus infection in South American wild birds. As part of a national surveillance program in Argentina, indigenous waterbirds were screened for antibodies to AI virus. From November 2006 to July 2007, serum samples from 540 waterbirds of 12 species were tested for type-specific antibodies to AI virus with the use of a commercially available blocking enzyme-linked immunosorbent assay (bELISA) and the agar-gel immunodiffusion (AGID) test. Thirty-three percent (176/540) of serum samples were positive with the bELISA and 12% (64/540) were positive with the AGID test. The bELISA detected antibodies to AI virus in eight of the 12 species, and the AGID detected positives in only five species. These results provide insight into AI virus circulation in Argentinean waterbirds and preliminary data to guide further surveillance efforts.