The effects of the juvenile hormone analog fenoxycarb (CAS No. 72490-01-8) were investigated in artificial indoor streams. The results from aufwuchs and two mayfly species ( Rhithrogena semicolorata and Ephemerella ignita) are presented. Four concentrations ranging from 0.05 to 50 μg/L (with a spacing factor of 10) were tested. Fenoxycarb disappeared rapidly from the water phase (DT 50≈5 days in the highest concentration, less in the other concentrations). Physico-chemical parameters and aufwuchs were not affected by fenoxycarb. The mayfly R. semicolorata, introduced at the start of the experiment, was affected by treatments of 5 and 50 μg/L. For the larvae in the streams a LC 50 of 3.3 μg/L and for the larvae in the enclosures a LC 50 of 2.5 μg/L were calculated. The second species ( E. ignita) was introduced 72 days after the application, at which time no fenoxycarb was detectable in the water of the streams (limit of detection of 0.5 ng/L). The emergence of E. ignita was affected in the highest treatment (50 μg/L). Ninety percent of the emerged imagoes showed morphological abnormalities at the abdomen.