The distribution and population structure of the neustonic isopod Idotea metallica were studied from samples collected on two consecutive plankton cruises performed in spring off the Catalan coast (western Mediterranean). On both cruises, high densities were observed along the oceanic side of the shelf-seas front, generally with lower numbers on each side of the front. High densities of the isopod were associated with samples with high quantities of flotsam. Size-frequency distributions were dominated by juvenile individuals, and no significant differences were found in size structure between the population on the oceanic side of the front and that on the coastal side. Temporal variability in the observed distribution patterns is attributed to the mesoscale variations in the inshore-offshore position of the shelf-slope front, which appears to act as a semipermeable barrier concentrating I. metallica, associated with other oceanic neustonic organisms, on its oceanic side.