The use of pyrethroid insecticides is increasing for agriculture, commercial pest control, and residential consumer use. In addition, there is a trend toward the use of newer and more potent compounds. Little is known about the toxicity of sediment-associated pyrethroid residues to aquatic organisms, yet recent work has shown they commonly are found in aquatic sediments in the heavily agricultural Central Valley of California, USA. Minimal data exist on the sensitivity of standard sediment toxicity testing species to pyrethroids, despite two or more decades of agricultural use of these compounds. Sediment concentrations causing acute toxicity and growth impairment to the amphipod Hyalella azteca were determined for six pyrethroids in three sediments, ranging from 1.1 to 6.5% organic carbon (OC). In order of decreasing toxicity of sediment-associated residues, the compounds tested were bifenthrin (average 10-d median lethal concentration [LC50] = 0.18 microg/g OC), lambda-cyhalothrin (0.45 microg/g OC), deltamethrin (0.79 microg/g OC), esfenvalerate (0.89 microg/g OC), cyfluthrin (1.08 microg/g OC), and permethrin (4.87 microg/g OC). In a sediment containing about 1% OC, most pyrethroids, except permethrin, would be acutely toxic to H. azteca at concentrations of 2 to 10 ng/g dry weight, a concentration only slightly above current analytical detection limits. Growth typically was inhibited at concentrations below the LC50; animal biomass on average was 38% below controls when exposed to pyrethroid concentrations roughly one-third to one-half the LC50. Survival data are consistent with current theory that exposure occurs primarily via the interstitial water rather than the particulate phase. A reanalysis of previously reported field data using these toxicity data confirms that the compounds are exceeding concentrations acutely toxic to sensitive species in many agriculture-dominated water bodies.