Predation by juvenile 1 + flounders, Platichthys flesus, (mean total length 8.0 cm+/-0.6 cm SD) on juvenile Macoma balthica (mean length 1.94 mm+/-0.40 mm SD) and Bathyporeia pilosa (mean length 4.11 mm+/-0.47 mm SD) was studied in a series of aquarium experiments. Prey vulnerability and consumption, as well as possible switching behaviour by the predator, were studied with varying total and relative prey densities. The functional response of the predator was also determined from the experimental results.The consumption and vulnerability of both prey species were equal when the prey species were presented separately to the predators (50 ind/aq). In the comparable two-prey-species experiment, where equal proportions of the prey species were presented to the predators, total prey consumption was about the same as in the single-prey-species experiment. Prey consumption during the night was clearly lower than prey consumption during the day with the same prey density. In all experiments, juvenile flounder showed a slight preference for Macoma before Bathyporeia independently of the relative or absolute prey density. No switching behaviour was observed. The consumption of both Macoma and Bathyporeia increased significantly with increasing total prey density. The functional response of flounder was in both cases of the type III. The best fit of the data was achieved with a continuous model of the type-III response.