(1) An isolated population of Nucella lapillus lives on Old Den, a pebble bank in the Humber Estuary surrounded by mud. This is an unusual habitat for Nucella for it is not normally associated either with shingle or mud. (2) Before 1962 Nucella relied on Balanus as a food source, but in the period 1962/64 Balanus was destroyed, and Nucella turned to Cerastoderma edule, a thinly-scattered prey washed on to the bank from the mud banks which surrounded it. (3) In 1965 Balanus resettled on the bank, and Nucella in the area resumed feeding on it. The settlement of Balanus induced a considerable emigration of Nucella from areas where it fed on Cerastoderma. (4) Nucella feeding on Balanus were more contagiously distributed than those feeding on Cerastoderma. (5) Nucella seeking Cerastoderma travelled between two and three times the distance travelled by Nucella seeking Balanus. This was associated with the dispersive behaviour of Nucella seeking thinly-scattered prey. (6) Nucella feeding on Cerastoderma were more rapacious than those feeding on Balanus. It was concluded that the rapacity of Nucella depended on the availability of food in the natural environment. (7) When given a choice, most Nucella preferred Mytilus as food. (8) Nucella accustomed to feeding on cockles in natural conditions readily consumed them in the laboratory, unlike barnacle-fed Nucella. This trait was partially lost when Nucella were provided with Balanus or Mytilus but was regained if cockles were provided in the absence of other food organisms. The mechanism involved with the acquisition of a taste for a new prey organism may be described as associative learning. (9) The efficiency of prey penetration in Nucella partly depends on the amount of experience gained by the predator.