Al-Naymi, N.A.Sh., H.A.S. AL-Nuaimi and M.R. Nashaat. 2022. Toxicity Stress of the Durah Power Plant Ash and its Effect on the Alga Chlorococcum humicola (Naeg) Rabenhorst 1868. Arab Journal of Plant Protection, 40(2): 188-192. https://doi.org/10.22268/AJPP-040.2.188192 This study illustrates the acute toxic effect of ash released from Durah power plant (DPP) on the biology of the phytoplankton species Chlorococcum humicola in Iraq. The results showed that the median lethal concentration for killing 50% of the Alga population (LC50) was 0.15 and 0.13 ppt (parts per thousand) for 24 and 48 hours exposure to crude ash concentrations, respectively. In contrast, no LC50 value was recorded for 72 and 96 hrs after exposure. The reduction in the optical density absorption value and the growth rate recorded was 0.083±0.121 cells for the highest ash concentration used, compared with 0.594±0.099 cells recorded for the control group. On the other hand, the doubling time for the control group was 1.16±0.652 an hour compared with 1.36±0.981 an hour recorded for 2 ppt ash exposure. The current study confirms that the crude ash concentrations tested had an adverse toxic effect on the biological parameters of the algal species Chlorococcum humicola in Iraq. Keywords: Algae, fly ash, pollutants, power plant, C. humicola