In-situ investigations on the life of the common sea star (Asterias rubens L.) were carried out in 1976, employing the Underwater Laboratory 'Helgoland' in Lübeck Bay (Western Baltic Sea). The abundance of A. rubens amounted to 2-31 m SUP--2 on sediment (fine sand), and to 324-809 m SUP--2 on mobile algal carpets drifting over the bottom. Actual population parameters (abundance, size class distribution) areinfluenced by both substrate quality and drifting. Stomach investigations revealed prey-size selectivity: small sea stars feed mainly on the snail Hydrobia ulvae when living on the sediment, but on mussel brood (Mytilus edulis) in the phytal. The principal food items of larger sea stars are the sand-dwelling clam Macoma baltica and the phytal-living isopod Idotea baltica respectively. A.rubens is very adaptive to the food availability; the diversity of its diet corresponds to the species diversity found in its environment. A change of biotope during active or passive migrations causes switching. The sea star is able to catch motile animals and to dig outinfaunal clams. It exhibits a diurnal feeding pattern related to light periodicity; the activity decreases at night. The average frequency of feeding is highly dependent on predator body size; it declines with growth. In situ-experiments indicate an exponential relationship between the feeding duration upon M. baltica and the quotient of clam size to logarithm of sea-star size. An approach is made toward a rough estimate of macrofauna consumption by A. rubens on sediment. The sea star seems to be an important predator and thus a competitor of demersal fishes on soft bottoms of the western Baltic Sea.