We quantified the prey selection and intake rate of a wading bird, knot Calidris canutus, when 6 different, intertidal prey species, the mud snail Peringia ulvae and the bivalves Macoma balthica, Cerastoderma edule, Mya arenaria, Scrobicularia plana and Mytilus edulis, were abundant. Knot usually search for food by randomly probing into the mud. Prey selection may be described by 5 rules: (1) Prey are not ingestible by knot if the circumference is more than 30 mm. (2) Prey are not accessible to knot when they lie buried deeper than 2 to 3 cm. (3) The probability of a prey organism being detected depends on its surface area, measured in the horizontal plane. (4) Prey are ignored when they are unprofitable, i.e. when the rate of intake while handling the prey is below the current overall average intake rate during feeding. (5) Knot prefer thin-shelled to thick-shelled prey species, possibly because a high inorganic content has an inhibitory effect on the rate at which energy can be extracted from the food. The first rule of ingestion is set by the gape width, and is therefore invariable. The fraction of the prey which is accessible varies according to the probing depth of the bird. The lower size threshold of prey taken is determined by the profitability rule and so varies according to the current feeding rate of knot. Unfortunately for knot, the majority of the preferred thin-shelled prey live out of reach of the bill, whereas the thick-shelled species live at the surface. Medium-sized Macoma balthica is the best compromise available in the 6-species mix.