On the basis of the generalization and systematization of the results of the regime observations made by environmental monitoring services it has been made an ecological assessment of the water quality in the Ukrainian part of the Western Bug River basin (in six section lines of observation). It has been found, the largest source of mechanical contaminations, organic pollutants and nutrients in the Western Bug River is the river Poltva. That is why insufficiently treated waste water of Lviv increases the level of water pollution in this Basin. Main focus is concentrated on the river Western Bug and its stream tributary (the Poltva river), which significantly affects its ecological status. The data concerning pollution indicators for 2015–2018 years was analyzed. A comparison characteristics of the change of pollution of the investigated water objects is carried out in three lookout points: in the river Poltva before falling into the river Western Bug, in the river Western Bug (background area) the above-fall of the Poltva River, in the river Western Bug the below-fall of the Poltva River.                 Exceeding of the permitted values of most indicators was detected in 2017: suspended matters – 2.4 MPC, ammonium nitrogen – 8.7 MPC, phosphates – 8 MPC, low level of dissolved oxygen is observed, because of high content of organic matters. This situation is better in the basin of the Western Bug River in the sampled area in 2018. The water of the Poltva River is much cleaner in comparison to the previous year, containing up to 2.3 times less suspended substances, 5.1 times less phosphates and 1.3 times organic substances. In this year there is a significant improvement in the ecological state of the Western Bug River, after the falling into the Poltva River. The ecological state of water of these water basins depends on the efficiency of the treatment facilities, especially in Lviv. Municipal wastewater treatment facilities have undergone repair works over recent years in Lviv.