The presence of an infaunal burrow structure facilitated the penetration of a medium fuel oil mousse into subsurface sediments to a greater depth than similar sediments lacking a burrow structure (destroyed by digging). The increased penetration of oil appeared to be related to better drainage characteristics resulting from the burrow structure. Oil was found to penetrate burrows of all sizes down to at least 1 mm diameter. Penetration of oil into the burrows occurred within 24 hours and was accompanied by a decrease in feeding rate of the lugworm Arenicola marina, although there were no detectable toxic effects of the oil on the small-sized infaunal community during the one-month experiment. Depending on the toxic properties of the oil, reworking of oiled sediments by the infauna may return oil to the surface, thus promoting degradation.