Insecticides are commonly used around homes for controlling insects such as ants, termites, and spiders. Such uses have been linked to pesticide contamination and toxicity in urban aquatic ecosystems. Fipronil is a relatively new and popular urban-use insecticide that has acute toxicity to arthropods at low-ppb levels. In this study, we collected runoff water from 6 large communities, each consisting of 152 to 460 single-family homes, in Sacramento County and Orange County, California, and evaluated the occurrence of fipronil and its biologically active derivatives over 26 months under dry weather conditions. Statistical modeling showed that the levels of fipronil and derivatives in the runoff water were both spatially and temporally correlated. More than 10-fold differences were observed between the Sacramento and Orange County sites, with the much higher levels for Orange County (southern California) coinciding with heavier use. The median concentrations of combined fipronil and derivatives for the Orange County sites were 204-440 ng L(-1), with the 90th percentile levels ranging from 340 to 1170 ng L(-1). These levels frequently exceeded the LC50 values for arthropods such as mysid shrimp and grass shrimp. The highest levels occurred from April to October, while decreases were seen from October to December and from January to March, likely reflecting seasonal use patterns and the effect of rain-induced washoff. Fipronil and fipronil sulfone (oxidation derivative) each accounted for about 35% of the total concentrations, with desulfinyl fipronil (a photolytic product) contributing about 25%. Results of this study clearly established residential drainage as a direct source for pesticide contamination in urban waterways, and for the first time, identified fipronil as a new and widespread contaminant with potential ecotoxicological significance.