Selenium is a trace element and an essential microelement for humans, animals and microorganisms, whereas this has not been established for plants yet. It is widespread in the earth's crust and occurs in soil, seawater and freshwater as well as minerals and sediments. Climatic conditions, geological composition of the soil and numerous anthropogenic factors affect the presence of selenium in the environment. It can exist in four oxidation states. Selenite is the most important form of selenium in soil. It can convert into selenate due to microbiological activity. Being the most soluble inorganic forms of selenium, selenite and selenate are the most accessible to plants. The effect of selenium depends mainly on its form and concentration in plants. We studied the influence of different concentrations and forms of selenium on biochemical and morphological characteristics of common duckweed. Plants were exposed to various concentrations (1 mg/L, 2 mg/L and 5 mg/L) of sodium selenite solutions (Na2SeO3), various concentrations (1 mg/L, 2 mg/L and 5 mg/L) of sodium selenate solutions (Na2SeO4) and to a combination of selenite and selenate with a 2 mg/L concentration. Furthermore, we used a control group cultured in growth medium with no Se added. During the experiment, we measured the photochemical effect of photosystem II (FS II) with the PAM 2500 fluorometer, the activity of the electron transport system (ETS) and observed with the naked eye the morphological changes (changes in frond colour, counting fronds and root degradation) as well as prepared the samples for the measurement of photosynthetic pigments (chlorophyll a and b, carotenoids) and anthocyanins. We have established that plants that have been exposed to concentrations (1 mg/L, 2 mg/L and 5 mg/L) of sodium selenate, (1 mg/L, 2 mg/L and 5 mg/L) of sodium selenite and the combination of selenite and selenate, grew and developed poorly under stress conditions. Consequently, their photochemical effect of photosystem II (FS II), activity of the electron transport system (ETS) and concentrations of photosynthetic pigments were lower. This study will improve the understanding of how different forms and concentrations of selenium affect the common duckweed.