The paper presents seasonal variability of phytoplankton biomass and species composition of the planktonic algae in newly created dam reservoir Wilkówka in Wilkowice. Additionally, the paper presents seasonal variability of chlorophyll a concentrations. The research was conducted in 2014 in two research points (W1 – the part of river Wilkówka inflow to the reservoir and W2 – the part of the reservoir dam). The values of phytoplankton biomass and concentrations of chlorophyll a were low, instead Cryptophyceae and Bacillariophyceae were dominated in the qualitative structure of phytoplankton.