Changes in species composition, richness, diversity, biomass (biovolume), and spatial community structure of a lotic periphyton community were observed under experimental acidification (mean pH 4.5) with and without aluminum, in troughs fed from an oligo-dystrophic creek (mean pH 6.7). in experimental and control troughs, the community was dominated by diatoms throughout the treatment. Achnanthes linearis was dominant in all troughs before treatment as well as in the control during the experiment. Eunotia pectinalis, which was rare in all troughs before treatment, became dominant in the acidified troughs. After 3 mo of acidification, algal cell density and biomass were respectively 30 and 40–70 times higher in the treated troughs as compared with that in the control. The Sow diversity (H = 1.45–2.30) characteristic of the control trough, increased to 3.3 in the treated troughs after 1 mo of acidification, but after 3 mo of experiment the diversity was slightly lower in experimental troughs. In the control trough, spatial organization of the community was characterized by monolayered arrangements during the whole experiment, in both acidified troughs (acid and acid plus aluminum) the community became multilayered (3–4 layers). The increased complexity of the community in the acidified troughs seemed to be related to a decrease in the abundance of grazers in the periphytic community, mainly the collector–gatherers Ephemeroptera and Orthocladiinae.