Synthetic pyrethroids readily partition from the aqueous to the solid phase in aquatic systems. Previous work has focused on pyrethroid partitioning to sediment matrices. Within many aquatic systems, however, other carbon-containing materials are present and can be critically important to certain invertebrate species and ecosystem functioning. For example, some invertebrates readily process leaf material, and these processes may represent an additional route of contaminant exposure. To our knowledge, estimates for partitioning of pyrethroids to these nondissolved organic matter matrices and associated toxicity have not been examined. The objectives of the present study were to examine variation in organic carbon (OC)-based partition coefficient (K(OC)) among three size fractions of particulate organic matter from sugar maple (Acer saccharum) leaf litter and sediments for the pyrethroid insecticide bifenthrin and to examine variation in toxicity to Hyalella azteca among bifenthrin-bound organic matter matrices and sediment. Log K(OC) of [(14)C]bifenthrin was greatest within sediment (6.63+/-0.23; mean +/- standard deviation throughout) and lowest in coarse particulate leaf material (4.86+/-0.03). The H. azteca median lethal concentration was 0.07, 0.11, and 0.15 microg/g OC for leaf material, sediment, and a 50% mix of leaf and sediment, respectively. Nonoverlapping 95% confidence intervals occurred between the leaf treatment and the leaf-sediment treatment. This pattern was supported in an additional experiment, and at 0.22 microg/g OC, H. azteca survival was greater in the leaf-sediment mixture than in sediment or in leaf material alone (F=29.5, p<0.0001). In systems that contain sediment and leaf material, both greater partitioning of bifenthrin to the sediment fraction and preferential use of leaf substrates may drive H. azteca survival.