ABSTRACT: The potential predation impact of jellyfish Aurelia aurita on zooplankton in a shallow cove was investigated. Population clearance potential was calculated from laboratory clearance rates and from measurements of medusae size and abundance during1991 and 1992 in the shallow cove Kertinge Nor, Denmark. Clearance by A.aurita as a function of medusae size and water temperature was measured in the laboratory using copepods (Acartia tonsa) and rotifers (Brachionusplicatilis) as prey. Clearance rate increased (from 0.01 to 2.3 l h^-1 ind.^-1) with increasing medusae diameter (from 4 to 44 mm) when rotifers were used as prey. For medusae &GT5 mm, clearance on copepods and rotifers wassimilar. Water temperature had marked effect on clearance, which increased from 0.16 l h^-1 ind.^-1 at 5*C to 0.58 l h^-1 ind.^-1 at 14 for a 20 mm medusa feeding on A.tonsa. Between 14 and 21*Cclearance was constant, and at 30*C the medusae died. In spring, when medusa were small, the population in Kertinge Nor could potentially have cleared the volume of water in the cove less than 0.1 times each day. This value was 3.5 times each day in latesummer when biomass was high. Blooms of rotifers were observed in Kertinge Nor, but only during periods of low population clearance potential of A.aurita, and the abundance of holozooplankton in general was extremely low in Kertinge Norduring 1991 and 1992. High predation pressure by medusae appears to control zooplankton in the shallow cove, at least during summer and early fall, and A.aurita may thus be considered as a keystone species in the control of the trophicstructure in Kertinge Nor.