This paper describes aspects of the winter feeding ecology of Black-tailed Godwits Limosa limosa on an intertidal mudflat in the Tagus estuary, Portugal. Their diet consisted mainly of the bivalve mollusc Scrobicularia plana, which represented 88% of the ingested biomass. The remaining 12% was the worm Nereis diversicolor and, occasionally, the snail Hydrobia ulvae. Ingested Scrobicularia varied between 10 and 20 mm in length, and all size classes of Nereis present in the sediment, except those smaller than 20 mm, were preyed upon. A change in dietary composition occurred over the winter, probably in response to changes in the availability of Nereis and small Scrobicularia. Intake rates averaged 74.9 mg dry weight per minute, which corresponded to a net energy intake of 1.21 kJ per min.