We examined the algal composition of an extremely acidic, metal-rich drainage pond of a pyrite mine located in southeastern Portugal. Two acidophilic diatom taxa, Eunotia exigua ("Sippenkomplex") and Pinnularia acoricola var. acoricola were found to be dominant at the site. The well known acidophilic taxon Euglena mutabilis was also abundant. Ulothrix tenerrima and Chlamydomonas spp. were also identified. Green jelly balls containing multiple diatoms and chlorococcal forms inside their assemblages were also present. These diatoms displayed greater vitality than their free-living counterparts, exhibiting more vivid colours. Rare specimens of an unidentified colourless flagellate, as well as Achromatium volutans, a large sulphur oxidizing bacterium, were also present. The algal species diversity of the S. Domingos mine acid drainage is very low and most of the taxa found can also grow heterotrophically on a variety of organic compounds, a clearly advantageous feature in this hostile environment.