The influence of increased fluoride concentrations generated by a wastewater treatment plant on the spatial distribution and abundance of benthic larvae of Hydropsychidae (Insecta, Trichoptera) species in the Cache la Poudre River (Colorado) was examined. Acute lethal concentrations of fluoride ion (F-) to these species were determined in soft water (average value of hardness 40.2 mg CaCO3/L) by static toxicity bioassays. The wastewater treatment plant caused a significant (P less than 0.05) increase in the fluoride concentration at three downstream sampling sites (mean values 1.17, 0.84, and 0.56 mg F-/L at 0.1, 1.6, and 9.2 km downstream sites, respectively) compared with the upstream reference station (0.31 mg F-/L). The 48, 72, 96, 120, and 144-h LC50s (mg F-/L) were 52.6, 25.8, 17.0, 13.4, and 11.5 for Hydropsyche bronta Ross, 102.0, 53.5, 34.7, 27.0, and 21.4 for Hydropsyche occidentalis Banks, and 128.0, 73.2, 42.5, 31.9, and 24.2 for Cheumatopsyche pettiti (Banks). LC50 values for H. bronta were significantly (P less than 0.05) lower than LC50 values for the other two test species. Abundance and biomass of all hydropsychid species were significantly (P less than 0.05) lower at the 0.1 km downstream site than at the upstream reference site. H. occidentalis was usually the most abundant species at the reference and 9.2 km downstream stations, with highest abundances at the 9.2 km downstream site. C. pettiti was dominant at 0.1 and 1.6 km downstream sites, showing higher abundances at the 1.6 km downstream site than at the upstream reference site. H. bronta was never collected at 0.1 and 1.6 km downstream sites, but was abundant at the upstream reference site.(ABSTRACT TRUNCATED AT 250 WORDS)