Many years of acid rain have caused countless lakes throughout the world to reach a pH too low to support any animal life and often only limited types of plant life. In this report, a new acidic algal culture medium, KC-1, was developed to simulate acid lake conditions in the laboratory. This medium has the ability to maintain a pH in the range 4.0 to 5.5 by the use of ammonium sulfate (NH4)2SO4 as its primary buffer. Three filamentous strains from two genera of the green algal family Zygnemataceae (Spirogyra communis and Mougeotia transeaui) were grown in KC-1 medium to observe their morphology at low pH levels (range 4.0 to 5.5). Morphology differed markedly for all three strains, when compared to morphology of these same strains that had been maintained in nonacidic culture for up to 30 years. Differences included variations in cell width, cell length, and the structure of cell walls and the chloroplasts. These characters are critically important in species-level taxonomy, which may be complicated by effects of acid rain on phenotype.