Currently, there is an ongoing process of development of the next-generation pesticides, which use for the purposes of agriculture has negative implications for the environment. Their hazard potential is attributable to two factors: high biological activity and their application method involving their dispersion in the environment. It necessitates the investigation of the effect of pesticides on aquatic organisms, which includes changes in their habitat, disruptions of their vital functions, especially at the early stages of their ontogeny, accumulation in their tissues, and movement along the trophic chain. Toxicological assessment of the fungicide Cayunis, EC effect has been conducted on the aquatic organisms of various taxonomic groups: microalgae (Scenedesmus guadricauda (Turpin) Brébisson, 1835), macrophytes (Elodea canadensis Michaux, 1803), molluscs ( Planorbarius corneus (Linnaeus, 1758)), embryos and fry of the round goby (Neogobius melanostomus (Pallas, 1814)). Over the course of the experiment, the conditions for intoxication were identified, and the toxicometric characteristics of the aquatic organisms belonging to various trophic levels were estimated. Using an experimental approach, the threshold and no observed effect concentrations of the preparative form of Cayunis, EC pesticide have been identified for each test subject. This study made it possible to compare the sensitivity of each investigated test subject to the fungicide. Among the studied test subjects, round goby fry has proved to be the most sensitive to the effect of Cayunis, EC fungicide.