The quantitative distribution of dominant phytoplankton species was carried out in the stabilisation ponds of Etueffont landfill leachate (Belfort, France) from May 1998 to May 1999. The results showed maximum phytoplankton abundance in the last lagooning basin during summer 1998, that coincided with an increase in temperature and better leachate quality. In addition, the phytoplankton was dominated by Euglenophytes species Phacus sp. and Euglena sp. known to resist constraints induced by the highly polluted aquatic environments. An improve in water quality of the last basins translated into a shift in species composition with the substitution of the Bacillariophyceae (Stephanodiscus dubius) by the Chlorophyceae (Coelastrum sp.).