Pyrethroid pesticides may move into surface waters after application through spray drift and stormwater runoff. These compounds are hydrophobic in nature and thus preferentially partition on to suspended and bedded sediment particles. Because of this association, sediments in urban streams and agricultural water bodies are considered the predominant "sink" for pyrethroids in California. Low concentrations of pyrethroids in sediments may be of significant ecological concern because pyrethroids exhibit toxicity to insects and other aquatic invertebrates that serve as the food supply of fish and other aquatic animals. A new method for the analysis of pyrethroid pesticides in sediment was developed using microwave-assisted extraction (MAE) combined with the Quick, Easy, Cheap, Effective, Rugged, and Safe (QuEChERS) method followed by liquid chromatography triple quadrupole mass spectrometry (LC-MS/MS). Mean recoveries for bifenthrin and permethrin by MAE were 37.1% (range 20-63.9%), and 60.0% (range 27.4-102%), respectively. To confirm the effectiveness of this method for environmental applications, sediments collected from four San Diego County streams were analyzed. At least one pyrethroid was present in 88% of the sediment samples. Bifenthrin was detected the most frequently (75%) with concentrations ranging from 11-202 _g/kg dry weight. Permethrin was detected less frequently in concentrations up to 420 _g/kg. Forester Creek contained the highest pyrethroid concentrations measured in the study. The limits of detection for bifenthrin and permethrin in sediment were less than 1.0 _g/kg and 5.0 _g/kg. Sediment pyrethroid concentrations were compared to published average 10-day median lethal concentrations (LC50, dry weight) for H. azteca (bifenthrin, 12.9 _g/kg; permethrin, 201 _g/kg). According to these values, concentrations of bifenthrin detected at Los os Peñasquitos Creek, San Diego River, and Forester Creek exceeded the LC50 values in eleven out of sixteen samples. Two samples from Forester Creek had concentrations of permethrin that were above the LC50 for H. azteca. These results suggest that bifenthrin in particular may pose a potential risk to sensitive benthic organisms in these water bodies. Any observed sediment-associated toxicity is less likely to be associated with permethrin. Additional research is needed to explore the actual risks that pyrethroid pesticides have on stream benthic communities.