The development of chlorophyll a is dependent on a number of physico-chemical factors, with high sensitivity to changes in water properties. Among the factors that influence the development of chlorophyll the most important are pH, temperature, solid suspensions, salinity, light, transparency, hydrology, human activities. The method for determining chlorophyll in this case was made using the submersible multiparameter ExoYsi 2, which uses the light spectrum of different wavelengths to determine chlorophyll and phycocyanin, a pigment found in blue algae. The sampling was carried out along the Danube from km 375, until the flow of the three arms into the Black Sea. A number of 23 points were selected along this length, which includes the eastern Pontic sector, the Predobrogean sector and the Deltaic sector. The determinations were performed in June, in the years 2019 and 2020, a period in which the course of the Danube usually has a tendency to reduce the flow. In the determinations performed along the Danube were identified low values of chlorophyll varying between 2.77 μg / L and 7.23 μg / L in 2019, and between 1.98 μg / L and 9.91 μg / L in 2020, values corresponding to class I quality according to the Order of the Minister of Waters and Environmental Protection 161 / 16.02.2006.