FLUCTUATING ASYMMETRY IN AQUATIC ORGANISMS AND ITS APPLICATION TO EVALUATION OF ENVIRONMENTAL IMPACTS. Symmetry refers to the regular arrangement of various body parts, which can form an axis that will divide the organism into two or more opposite sides, each of them being the mirror-image of the other. Bilateral symmetry is the most frequently observed type of symmetry in the nature, and the parts of a bilateral organism are oriented by the orthogonal intersection of the anterior-posterior and the dorsal-ventral body axes. Over time, it was observed in some cases that not only symmetrical morphological characteristics were present, but also asymmetrical traits, where one part that should be symmetrically identical to other presents some differences. Industrialization and urbanization led to dramatic changes to many natural ecosystems, with side-effects on the biology of different species, and on the structure and dynamics of organisms populations and communities. The evaluation and correct interpretation of the alterations caused demand sensitive methods to detect changes at different levels: individuals organisms, species, populations and communities. The observation of changes to the process of development of an individual would be a way of evaluating alterations, variations and fl uctuations to symmetrical characteristics, and these might be indicators of environmental health. The appearance of structural assymmetry, morphological deformities and malformations caused by anthropogenic changes to the environment and/or pollutants are used in monitoring environmental quality. The aim of this study was to present and discuss concepts and issues on biological fl uctuating asymmetry and their utility as tools for evaluating environmental quality. Some theoretical concepts and evolutive aspects of biological fl uctuating asymmetry are included, as well as an overview of their use in environmental diagnosis and further implications. Keywords: fl uctuating asymmetry, biomonitoring, Bilateria, pollution, environmental quality.