The White-rumped Sandpiper (Calidris fuscicollis) breeds in Alaska and Canada, and during the nonbreeding season migrates to South America. The reported diet of this sandpiper is principally invertebrates; seeds are a very rare item. This species was studied at Guaminí Pond, Buenos Aires Province, Argentina, in December 1997. The stomach contents of 23 adults were investigated. The analysis showed that during the sampling period the only food eaten by the White-rumped Sandpiper were seeds. Them were represented by the following families: Caryophylaceae, Chenopodiaceae, Fabaceae, Malvaceae, Polygonaceae, Ulmaceae, Cyperaceae and Poaceae. The seeds ranged between 0.83 and 2.25 mm in length. Stones and fragments of algae were found in all samples, the algae possibly ingested accidentally during feeding. Our findings indicate that the White-rumped Sandpiper forages on different items depending on the area, season and availability, and may be an opportunistic feeder.