For a better understanding of the behaviour of90Sr in aquatic systems, this β-emitter has been determined in the different layers of a lake sediment core which has been previously dated using the well established210Pb dating method. For this, a radiochemical technique has been developed for the isolation of strontium from the sediment samples and for its β activity measurement with a low-level gas-flow proportional counter. A conservative behaviour of90Sr in the water column remaining dissolved in the water body, can be inferred from the data obtained. Consequently, a possible90Sr transport pathway through the outflowing waters of the lake can be deduced.