Biota-sediment accumulation factors (BSAF; concentration in organism lipid/concentration in sediment on an organic carbon basis) of polyaromatic hydrocarbons varied with season and along an intertidal gradient in a coastal marsh in San Francisco Bay. The BSAFs were lowest during the local rainy season. During the dry season, BSAFs were lowest in the high intertidal zone closest to shore. Significant differences among species groups were also observed; BSAFs were lowest in polychaetes and highest in the asian clam (Potamocorbula amurensis), varying over almost three orders of magnitude (0.0069-5.4 g sediment organic C/g lipid). The BSAFs decreased with increasing percent fines in the sediments and with PAH concentrations on an organic carbon basis. We suggest that a determining variable is the content of highly aromatic soot particles, which increases during periods of surface runoff and which is expected in the dry season to be highest in the high intertidal zone where these finer particles preferentially accumulate. Correlations of BSAFs with the ratio of the logarithm of the activity coefficients in porewaters to those in sediments were generally stronger than with log Kow, indicating a limitation of octanol as a surrogate for sediment organic carbon or organism lipid. These observations qualify but also strengthen the concept of equilibrium partitioning as the determining factor in bioaccumulation by benthic organisms of nonpolar organic compounds from sediments; the assumption that “organic carbon” can be considered in generic terms without allowance for aromaticity and probably other factors as well, must, however, be reconsidered.