The phenylpyrazole insecticide fipronil and its degradates are a potential surface-water contaminant and toxicant to nontarget species such as aquatic macroinvertebrates. To better understand how fipronil, fipronil sulfide, fipronil sulfone, desulfinyl fipronil, and fipronil amide affect aquatic communities, a 30-day mesocosm experiment was run. Rock trays were colonized with natural benthic communities in the Cache La Poudre River in the mountains of northern Colorado and transplanted into a laboratory experimental stream setting. In total, there were 36 experimental streams: 3 controls, 3 solvent controls, and 30 treatments. Water quality metrics and samples for pesticide analysis were collected throughout the experiment. At the end of the experiment, larval invertebrates remaining in each experimental stream were collected, enumerated, and identified. Emergent insects were collected each day of the experiment and identified to lowest taxonomic unit. These data were used to derive species-specific effect concentrations and, along with published data, derive species sensitivity distributions for fipronil(s) and hazard concentrations for the 5th percentile of affected species (HC5). The resulting HC5 values were used to convert fipronil compound concentrations in field samples to the sum of toxic units (∑TUFipronils), and the field invertebrate data were converted into a Species at Risk (SPEAR) pesticides metric (SPEAR_pesticide) and used to explore the relationship between the invertebrate community and ∑TUFipronils.