The interactions between the form of a barnacle aggregation, its flow environment, and the feeding behavior of each individual was determined in unidirectional flows; both models of barnacle aggregations and live barnacles were used. Hill-shaped aggregations of model barnacles captured significantly more particles than flat aggregations. In general, rows upstream of, and at the peak of, all hill-shaped profiles captured significantly more particles than downstream rows. Living barnacles located at, or upstream of, the peak of natural clusters captured significantly more food particles than did barnacles located downstream. Living barnacles located at, or upstream of, the highest point in a natural cluster fed passively, whereas barnacles downstream of the peak actively swept their cirral net against the flow. Flow was laminar up to the highest point in natural clusters, whereas flow was both reduced and turbulent over the downstream portions. Individual barnacles within a cluster differ in their feeding rates and net energy gains, and therefore differ in their growth such that, in unidirectional flow, the peak of a cluster will shift upstream over time; in oscillating flows, the clusters will develop a symmetrical profile.