This study, estimates the predicted no effect concentration (PNEC) for the protection of organisms in aquatic and soilenvironments, considering the mode of action of Perfluorooctanesulfonic acid (PFOS). PNECs were derived using the speciessensitivity distribution (SSD) approach to estimate the hazardous concentration for 5% of species (HC5), with applying assessmentfactors. Chronic toxicity data on PFOS were collected through the USEPA's ECOTOX database and literature reviews, andclassified by toxicity endpoints. PNECs were then derived for each of seven toxicity endpoints that met the criteria for SSDfitting. For aquatic organisms, the PNEC for PFOS, based on all available chronic toxicity data, was determined to be 0.53 μg/L. The PNECs for development, genetics, enzymes, growth, reproduction, population, and biochemical biomarkers were 0.28, 0.43,0.83, 0.90, 2.17, 111.17, and 3.53 μg/L, respectively. The lowest PNEC was observed when the toxic endpoint was set asdevelopment, which is considered to be due to the mode of action of PFOS, known to cause developmental toxicity by disruptingthe endocrine system of organisms. For soil organisms, toxicity data were insufficient to estimate PNECs for individualendpoints, so all available data were used to estimate a PNEC of 0.75 m...