The aim of this study was to assess a trophic state of water in Siemiatycze dam reservoir. This object is located in Podlaskie voivodship in the municipality Siemiatycze. Studies were carried out systematically once pursued in seven measurement-control points in 2014 year. The selection and placement of points in a longitudinal section of the reservoir was based on the need to capture the changes occurring in the studied indicators. In the researches there were used available in literature Trophic State Index, based on made measurement of the concentration of chlorophyll "a", total phosphorus and total nitrogen. Research results clearly show that concentration of total phosphorus and TSI(TP) index had greatest impact on a trophic state of water in Siemiatycze reservoir, which allowed to classify water to hypertrophy. The values of all calculated Indexes point to the fertility of the water in the studied reservoir. Any actions in order to protection should be taken, first of all by the reducing the impact of point sources’ pollution from wastewater treatment plants and tourist objects.