ABSTRACT The n-alkane fraction of Prudhoe Bay crude oil dispersed with Corexit 9527 disappeared rapidly from the water column directly beneath a seawater surface slick contained by an enclosure. Biodegradation appeared to be more important than nonbiological volatilization or photooxidation processes. Extracellular polysaccharides, synthesized by floc-forming bacteria, accumulated in the water column of the enclosures during n-alkane degradation. The study demonstrates the possible usefulness of Corexit 9527 in accelerating the conversion of this crude oil fraction by the indigenous microflora of coastal marine waters.