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The paper is based on the classification of river waters into salmonid and cyprinid. The main objective is to assess the compliance of Lithuanian river water temperature with the ecological requirements, referring to the temperatures mentioned in the organic classification. Data of river hydrological stations for the period 2003-2007 were used to evaluate water temperature. We measured what average decimal pointed and maximum water temperatures correspond to the temperatures mentioned in organic classification; river sections were grouped according to relationship between the average and maximum temperatures nature; organic temperature date of different river section groups. After various calculations, the rivers were divided into three groups according to the slope and the average temperature, and it was shown that correlation coefficients of the river groups are in the range of reliability when set significance level is 0.01 (1%). According to the provisions of the Freshwater Fish Directive, the water surface quality was divided into five classes: 1) very good, 2) good, 3) satisfactory, 4) poor, and 5) very bad. The data analysis of river waters and the average and maximum temperatures of a five-year period (2003-2007) showed that from the hydrological stations of 47 rivers in 14 water quality was very good / good, in 28 satisfactory and in 5 poor. Very poor water condition was not found in the analyzed rivers. The examination of the river groups crossing over 18 °C, 21.5 °C and 24 °C temperatures revealed that each of the river groups showed that the crossing dates are available at different times as the rivers have been classified in accordance with the slope values and temperature values. [ABSTRACT FROM AUTHOR]