Milkfish (Chanos canos) and tilapia (Oreochromis mossambicus) were exposed to varying concentrations of rotenone (95-98% active ingredient) to determine the median lethal concentration (LC⁵⁰). The 1, 6, 12, 24, and 96 h LC⁵⁰ were 64, 36, 36, 30 and 25µg/1 for milkfish and 172, 123, 91, 86 and 80 µg/1 for tilapia, respectivly, at 28±1^oC. In another experiment, O. mossambicus were exposed to a lethal concemtration of rotenone (100µg/1) that had been allowed to age for selected periods of time to determine whether significant degradation occurs within 24h. Mortality after 3 to 24 h exposure in 0 (no aging), 3 and 6 h aged rotenone solution were not significantly different from the control test even after 24 h of exposure. These results inducate that the treated water was no longer toxic to the fish after 12 h and acute toxicity of rotenone ceased towards the end of 12 h at 27 ± 1^oC