The location and orientation of epizooic barnacles on the surface of their host is crucially important to the survival and growth of the animal and is determined at the time of settlement by the cyprid larva. The present study examines the location and orientation of Chelonibia patula barnacles on the upper surface of the carapace of the crab Portunus pelagicus from the Mediterrannean coast of Israel. Cirral activity behaviour patterns, such as turning angles of the cirral fan, types of cirral motion, and beat duration were determined with respect to the angular deviation of the rostro-carinal axis (RCA) of the barnacles from the direction of the incoming flow. While small barnacles (up to 1 cm rostro-carinal length) perform normal feeding beats and turn the cirral fan towards the incoming flow at all angles up to 180°, larger barnacles (>1 cm rostro-carinal length) do not extend their cirri when oriented at RCA angles exceeding 120°. Nearly all the sampled barnacles are oriented on the carapace at RCA angles of no more than 120° with respect to the anterior margin of the host crab, which is the predominant direction of incoming flow and food. It is concluded that larval choice of orientation at settlement is determined according to the direction of the current and the anticipated food-gathering ability of the adult barnacle.