To determine the chronic toxicity of surface runoff water of the Kuyalnitskiy estuary, a toxicological bioassay of waterwas carried out every two months during 2019 from several sampling points from the catchment area: a watercoursefrom the territory of the Kuyalnik sanatorium, a watercourse of the Korsuntsovskaya beam from the pond system inthe Korsuntsy village, and a watercourse near the Krasnosilka village (beams Gildendorfskaya) and small rivers BolshoyKuyalnik, Kubanka. Chronic bioassay was carried out using a laboratory culture of Daphnia magna Straus (Cladocera,Crustacea), specially adapted to increased water mineralization. A criterion of chronic toxicity was a reliable decrease infertility indicators (number of juveniles) in the tested water compared with the control during the experiment.A statistically significant difference between the fertility indices in the experiment and control was noted duringbioassay of water from the watercourse of the Gildendorfskaya beams, watercourse from the Korsuntsovskiy pondssystem and watercourse from the territory of the Kuyalnik sanatorium. As a result of bioassay of surface waters of smallrivers Bolshoy Kuyalnik and Kubanka, chronic toxicity was not detected.The low quality of the surface waters of small watercourses and chronic toxicity are largely due to the locationof local pollution sources (landfills, bypass road) and sewage discharges in the southern part of the Kuyalnitskiy estuary.The obtained results coincide with the analysis of hydrochemical indicators of watercourses and reservoirs in the southernpart of the Kuyalnitskiy estuary catchment, the ecological state of which is assessed as “moderate” with a “moderatelypolluted” water purity.The detected chronic toxicity of water in small watercourses indicates their chemical pollution, which can negativelyaffect the development of the early nauplial stages of Artemia partenogenetica, for which the presence of locallydesalinated water areas in the coastal zone of the estuary is extremely important.