Quantitative field studies (density, wet weight biomass, Shannon diversity, species richness, evenness of distribution) on benthic sublittoral macrofauna were made in the vicinity of an oil refinery in southwest Finland before and after the installation of a new wastewater treatment plant that reduced the amount of oil and liquid effluents by ca. 90–95%. The number of species and species diversity increased during the 1st and 2nd yr after pollution abatement at the stations close to the former outflows. The amphipods Pontoporeia affinis, Corophium volutator, and C. lacustre, midge larvae of the Chironomus plumosus-group, the oligochaete Tubifex costatus, the polychaetes Harmothoe sarsi and Polydora redeki, and the bivalve Cardium sp. were the most successful recolonizers of the 23 taxa sampled. The strong lethal effect of oil-contaminated sediments upon Chironomus plumosus larvae decreased markedly in laboratory experiments (LT50 was estimated at 7 d in 1973 and at 28 d in 1974; in 1975, 80–90% of the larvae survived for 28 d). Details of postabatement succession are discussed. The results demonstrate not only the recovery from chronic oil pollution but also the degree of ecological damage caused by previous continuous discharge of oil. Key words: Baltic Sea, Chironomus plumosus, macrobenthos, oil pollution, pollution abatement, recolonization, sediment biotests