A microcosm experiment was carried out to determine the effects of the activity of the burrowing polychaete Nereis virens (Sars) on the associated meiofauna. The sediment basin (76 x 41 cm) was filled with 10 cm of sandy sediment previously sieved with a l-mm mesh to remove any undesired macrofauna and macrodetritus. Fifteen 13-cm long polyvinyl-chloride (PVC) tubes (I.D. = 10 cm) were pushed into the sediment to partition treatments. Nereis were added to the tubes at two densities, low (N = 1) and high (N = 3). Five tubes were: used as controls (no Nereis), while two sets of five tubes were used for the low (L) and high (H) density treatments, respectively. After 14 days, meiofauna was sampled by coring. Cores were cut into three slices: surface (0-1 cm), subsurface (1-5 cm), and deep (5-10). High densities of Nereis (H) significantly affected nematodes, harpacticoid copepods, and nauplii abundance. However, lower abundances were found only in the top cm of the sediment. Moreover, a significant number of dead nematodes found in this sediment layer of treatment H allowed a distinction between sediment disturbance effects and predation effects. Sediment disturbance caused by Nereis may be related to an intensive "ploughing" of surface sediment during food-searching activity. Diversity indices were affected only in the top cm of the sediment with generally lower values in treatment H. Differences in the relative survival of the different feeding groups were found in treatment H, where microvores and deposit feeders respectively showed greater and lower survival. Multivariate analysis (multidimensional scaling) revealed significant differences in nematode species composition among treatments in all sediment layers. It is concluded that N. virens significantly affects meiofauna mostly by disturbance of the top cm of the sediment where its predation represents an influent force as well. The structure of nematode assemblages in subsurface and deeper sediment layers is also affected, most likely by changes in redox conditions caused by the bioirrigating effects of Nereis burrows.