Field and laboratory studies on the morphology, gut contents, ingestion rates and digestion efficiency of Pollicipes pollicipes were combined to obtain estimates of the likely range and quality of materials required to sustain this species . Orientation of P. pollicipes on the shores of south; west Portugal appeared to be determined by microtopography. Animals generally faced into the wave backwash. Orientation could be temporarily altered by torsion of the peduncle in response to changes in flow direction, permitting more efficient filter feeding. Cirral and mouthpart morphology suggested that P. pollicipes, Capitulum mitella and Lepas anatifera were omnivores. Size-related changes in cirral morphology made small juveniles better equipped than adults to feed on small particles. Cirral activity of P. pollicipes was investigated. Very slow rhythmic cirral extension (or 'beating') was observed in all P. pollicipes, J but only in relatively still water; once flow rates exceeded 14 cm s all barnacles exhibited prolonged cirral extension. The 'beat' rate was temperature dependent in most animals and larger animals exhibited a lower extension frequency than juveniles. It was concluded that 'beating' was primarily respiratory in function and not a feeding mechanism. The gut contents of wild P. pollicipes included animal and algal material but little inorganic matter, -.,. Small organic material predominated in small juveniles while large organic material predominated in adults. The rates of faecal production and growth were much higher in barnacles feeding on zooplankton than on algae and although algal cells were ingested in high numbers, the energy intake was so low that animals barely maintained their body weight. Digestive efficiencies varied with diet but little with barnacle size. A wide range of digestive enzymes were identified in P. pollicipes and L. anatifera suggesting that a variety of foods may be digested. Specific enzyme activity was low, characteristic of more carnivorous animals