A dynamic 28-day toxicity study was conducted to determine the median lethal concentration (LCse) of Santoflex13 to fathead minnows (Pimephales tromelas). A flow-through proportional diluter system was used to maintain constant test concentrations. Exposure concentrations of Santoflex 13 were measured extracting a sample aliquot with methylene chloride and injection of a portion of the extract into a gas-liquid chromatograph equipped with a flame ionization detector. The mean measured levels were: 0.020 mg/1, D.C34 ag/1, C.089 mg/1, C.26 mg/1 and 0.92 mg/1. Nominal concentrations were: 0.066 ag/1, 0.12 mg/1, 0.23 mg/1, 0.45 mg/1 and 1.0 mg/1. All con- centrations were corrected for recovery for each appropriate sampling day.