The neustonic isopod Idotea metallica inhabits objects drifting at the sea surface. Animals found on floating patches represent not just ephemeral assemblages but persistent local populations. Drift material collected in the Mediterranean, the North Atlantic, and the North Sea harboured populations of up to about 50 animals including all developmental stages. In laboratory experiments the species proved to be able to establish populations on spatially limited, isolated substrates. The capacity of 5-litre-microcosms for I. metallica was about 130 animals. In the presence of the coastally distributed congener Idotea baltica, however, laboratory populations of I. metallica went extinct within 12 weeks. Even though high colonisation rates can be expected in coastal waters because of high patch densities, metapopulation persistence is mostly restricted to the open sea. In coastal waters extinction rate of local populations increases because of patch destruction and the species' inferiority to coastally distributed competitors. Due to high uncertainties in estimating patch densities, it is difficult to determine the parameters underlying metapopulation dynamics such as the migration rate and the rate of patch occupancy. © Springer-Verlag and AWI 2002.