Biodegradation of petroleum and phenol contaminations in the soil and water habitat is carried out by various organisms such as fungi, yeasts and bacteria. Quantitative analysis of petrolytic and phenolytic bacteria can be used for assessment of these pollutions. The research was carried out in the framework of comprehensive monitoring of the Lower Prut River Basin. The sampling was performed in the riverine stations Goteshti upstream, Goteshti downstream, Cahul, Slobozia Mare, Giurgiuleshti, and in the lakes Manta and Beleu. The cultivation of petrolytic and phenolytic microorganisms was carried out on selective media at 25°C in incubator INCUCELL. Water quality was assessed according classification of Vinogradov et al. (2001): class 1a - very clean, 2a - clean, 3a - moderately clean, 4a- polluted, 5a - dirty. The quantitative account of petrolytic and phenolytic bacteria for the Lower Prut Basin monitoring stations are shown in Fig. 1, where seasonal dynamic of studied groups of bacterioplancton are clearly seen. The seasonal rising of water temperature stimulates the development of microorganisms. But in this case, the correlation between the temperature and the number of bacteria was observed just before the start of high spring water in the Prut River that took place in April. Figure 1. The density dynamic of phenolytic and petrolytic microorganisms in the Lower Prut River Basin in January – April 2015 In spite of rising water temperature in April, the number of examined groups of microorganisms has fallen sharply along the watercourse. However, the local point of petrol pollution was registered in the station Cahul. The most significant density of petrolytic bacteria was registered downstream the station Goteshti in February, when the water level in the Prut River is the lowest. In accordance with the used classification, the water quality of the Lower Prut River can be attributed to the 3a Water Quality Class, corresponding to the status "moderate"