The head and jaw movements involved in capture, buccal manipulation, ingestion and rejection of prey were investigated using sequential photography of juvenile Atlantic salmon feeding in a simulated stream environment. The results are described and discussed and mouth breadth and gill raker spacing are proposed as morphometric limitations to the range of prey sizes available which remains constant at 0·06 · fish fork length (PFR).A recirculatory flume tank was used to study prey size selectivity behavior. Simplified downstream‐drifting prey items elicited a variety of responses depending on their physical size. One hundred percent of offered prey ofPFR0·025 were ingested, while 90 % of prey at PFR 0·051 and 100% of prey atPFR0·105 were rejected. It is demonstrated that fish show negative selection for prey sizes smaller thanPFR0·025 and that prey of this size elicits maximum growth response.The validity of the proposed morphometric limitations on the available prey sizes is demonstrated by reference to selectivity behaviour and prey size related differential