Planktonic microorganisms are an integral component of all aquatic ecosystems and their role in the transformation of autochthonous and allochthonous organics is difficult to overestimate. Their quantitative development is a reliable indicator of both the trophic status, and the sanitary and biological state of the water body. A quantitative analysis of the state of bacterioplankton at the mouth of the Prut River (Giurgiulesti) and in two branches of the Danube (Sulina and Sfântu Gheorghe) was done in the spring of 2019 (Table 1). The water quality of the lower part of the Prut River (Moldova) and the Romanian sector of the Danube delta was assessed based on the Moldovan Regulation on environment quality requirements for the surface waters (Government Decision no. 890/2013). Table 1. Microbiological indices and sanitary-microbiological state of the lower sectors of the Prut and Danube rivers, spring 2019 Sampling point Ntot (million cells/ml) Nsapr (thousand cells/ml) Bacterial index (Nsapr/Ntot), % Water quality class* Giurgiulesti 1.2 5.8 0.48 II-IV Sulina 0.8 0.92 0.12 II-III Sfantu Gheorghe 0.9 0.78 0.087 II-III Sacalin island 2.7 0.69 0.027 II-III * Water quality classes: II – good, III – moderately polluted, IV – polluted. According to obtained data, the water quality in the studied sections of both rivers, at least during the observation period, was relatively good and varied, in most of cases, within II-III (good – moderately polluted) classes, and only sometimes – within II-IV (good polluted) classes. For comparison, in spring of 2014 and 2015 at Giurgiulesti, the total number of bacterioplankton (Ntot) ranged 0.7-1.40 million cells/ml (water classes I-II), and the number of saprophytes (Nsapr) – 0.26-4.24 thousand cells/ml (classes II-III). In the same period, at Vilkova sampling points (Danube, lower part of the Chilia branch), Ntot was of 3.0-8.0 million cells/ml (classes III-IV) and Nsapr – of 2.18-35.42 thousand cells/ml (classes III-IV). The bacterial index varied at Giurgiulesti from 0.04% to 0.30% (classes I-III) and at Vilkova -from 0.027 to 1.1%, that means the water quality oscillated between the very good and very polluted (classes I-V).