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Several countries, including the United States, have agreed to destroy stockpiled chemical warfare agents in accordance with the Chemical Weapons Convention Treaty of 1993. Sarin is one of many chemical warfare agents (CWA) designated for destruction. In the event of an accident during incineration, sarin or its decomposition products have the potential to be expelled into the environment. Sarin hydrolyzes into isopropyl methylphosphonic acid (IMPA), a compound detected in groundwater from prior CWA production. This study determined the acute toxicity of IMPA to golden shiner, Notemigonus crysoleucas, and channel catfish, Ictalurus punctatus, eggs and 15-posthatch (dph) fry. The median lethal concentration (LC50) values at time of hatch for golden shiner and channel catfish eggs were 66.6 mg/L (hatched in 72 hr) and 167.5 mg/L (hatched in 168 h) IMPA, respectively. The 96-h LC50 estimates for 15-dph golden shiner and channel catfish fry were 93.9 and 144.1 mg/L IMPA, respectively. The lowest LC50 value from the most sensitive species in this study is approximately 100 times greater than the human adult lifetime drinking water health advisory value, and is approximately 2500 times greater than the critical reporting limit (> or =0.025 mg/L) for IMPA detection in groundwater from CWA production. These results are critical in understanding the toxicological properties of this potential environmental contaminant.