Algal flora of four post-mining ponds in the area of an abandoned pyrite mine (SW Poland) was examined for the first time. All algal taxa were identified and documented by light microscopy (LM) and scanning electron microscopy (SEM). Two types of waters (on the basis of their properties) were found: (i) strongly acidic with low pH (<3) and high concentrations of sulphates, Al, dissolved silica, and trace metals (e.g. Fe, Mn, Cu, Zn, and Co), as well as (ii) moderately acidic with pH between 4.0 and 4.8, and containing low concentration of ions, Al, dissolved silica, and much lower trace metal concentrations. All together, only 26 taxa (diatoms, filamentous green algae, euglenoids, dinophyte and chrysophyte stomatocysts) were recorded in the investigated acidic waters. Eighteen of them were found for the first time as associated with acid mine drainage (AMD), Eunotia exigua (Bacillariophyceae) was the most abundant alga in all studied ponds. The pH of water seemed to be the most important factor influencing species richness of algal communities in the area studied, as the number of taxa was twice as high in the moderately acidic waters (23) compared to the strongly acidic ones (11). The temperature of water, high concentrations of Fe, Al and trace metals, as well as the deficiency of nitrates and phosphates could be other selective factors for algal development in the AMDs studied.