Abstract <p>The main components of adaptive-compensatory processes in aquatic test organisms during the quality biotesting of an aquatic environment are considered. It is shown that adaptation is a set of physiological and biochemical responses in aquatic organisms that underlie the adaptation of an organism to changing environmental conditions and are aimed at maintaining relative constancy in its internal environment—homeostasis. It is emphasized that the study of adaptation processes provides an objective interpretation of biotesting results. The physiological meaning of the adaptation of aquatic organisms to external environmental impacts consists just in maintaining internal homeostasis and, accordingly, the viability of an organism in almost any conditions, to which it is able to respond adequately. Therefore, the functional systems of an organism work as self-regulated levels of the nervous and humoral systems, the action of which is aimed at achieving certain adaptive results beneficial for an organism. The leading role in this self-regulation belongs to the feedback processes, which were formed and fixed during the evolution of living beings as necessary for the survival of species. In other words, when the effect of physicochemical factors of an aquatic environment on the organism of hydrobionts occurs, compensatory and adaptive processes are launched in an organism to provide its survival in these conditions. This mechanism underlies the procedure of the quality biotesting of an aquatic environment.</p>