The effect of short-term and continuous acidification on the primary production and biomass of periphyton was studied in seminatural conditions using an experimental device simulating streams ecosystems. After allowing periphytic algae to colonize unglazed ceramic tiles for 4 to 6 weeks under natural conditions, the pH of the water was lowered with sulfuric acid (H2SO4) from 6.5 to 6.7 to 4 to 4.6 for short periods (12 to 72 h) or long periods (continuous acidification, 84 days). During short-term acidification, in all the experiments but one, there was a significant increase (up to 2.6 times as compared to the control) of the primary production and specific activity in the first 12 h, followed by a significant decrease and slow recovery from 48 h until the end of the experiment. If acidification is continuous, primary production and specific activity are significantly higher (between 10 to 15 times that of the control) in acidified channels, either with or without aluminum, by Day 33 and until the end. Biomass stayed constant or decreased in short-term acidification and increased (from 10 to 15 times) in continuous acidification.