Organic matter in sediment is derived from many sources, including dead plants and animals, fecal matter, and flocculated colloidal organic matter. Chemical partitioning and toxicity of nonpolar organic contaminants is strongly affected by the quantity of sediment organic matter. The purpose of the study was to determine whether the quality of sediment organic matter affects partitioning and bioavailability of such contaminants. A base substrate, amended to a consistent organic carbon level (nominally 0.4%, measured 0.35% + or - 0.11 sd POC (particulate organic carbon)) with five types of organic matter (a macrophyte, fecal matter of two invertebrate species, suspended particulate organic matter, and an organic-rich mud), was spiked with serial concentrations of the polynuclear aromatic hydrocarbon, fluoranthene. After a five-week equilibration period, the toxicity of the spiked substrates to an infaunal amphipod, Rhepoxynius abronius, was measured, and the distribution of fluoranthene between particulate and interstitial water phases (including total and freely dissolved interstitial water phases) was determined.