Chiral pesticides currently make up 25% of all pesticides used, and this ratio may further increase as more natural product-like compounds are introduced. Chiral pesticides consist of enantiomers that may have contrasting toxicological characteristics and may also be selectively degraded in the environment. The enantioselectivity in these processes can result in enhanced or reduced ecotoxicological risks that cannot be predicted from our current level of understanding. The main objective of this project was to evaluate enantioselectivity in biodegradation and aquatic toxicity for synthetic pyrethroids and chiral organophosphates, and to understand the role of enantioselectivity in regulating the overall environmental risk of these compounds. In this study we 1) developed separation and identification methods for analysis of chiral pesticides, 2) evaluated occurrence of enantioselectivity during pesticide degradation in soil and sediment phases, and 3) using bioassay and biochemical methods, characterized enantioselectivity of pyrethroids and chiral organophosphates in their acute and chronic toxicity to aquatic organisms. Findings from this study will provide not only data for assessing the enantioselectivity for these broadly used chiral insecticides, but also basis for advancing our understanding of environmental implications of chiral contaminants in general.