Isoetes bolanderi dominates the littoral flora of Siesta (elevation 2,440 m) and Ellery (2,905 m) lakes in the Sierra Nevada Range of California, USA. Both lakes are sparsely vegetated and I. bolanderi maintained aboveground oven dry weight of 30–50 m⁻²² through most of the 1981 summer growing season. Plants at the higher elevation Ellery Lake were half as large as plants at Siesta Lake and had substantially more biomass in corms. Titratable acidity levels in Isoetes leaves showed a diurnal fluctuation <50 μeq g¹ fresh weight early in the season at the highest elevation site but this increased to ∼300 μeq g¹ FW by mid-summer; starch and chlorophyll levels likewise increased in the leaves over this time. Throughout the season the magnitude of the diurnal acid change was comparable inIsoetes from both lakes but the dynamics of daytime deacidification were not. Averaged over the season, total daytime deacidification at Ellery Lake was 65% complete by noon whereas at Siesta Lake it was only 22% complete by noon. It is suggested that this may be related to the fact that Siesta Lake was more acidic and thus more carbon was in the form of free CO₂. In both lakes water chemistry showed no consistent diurnal fluctuation in pH or free CO₂ though total inorganic carbon levels were at the extreme low end for aquatic habitats. The studies reported here suggest that under extremely low inorganic carbon levels there may be selection for nighttime CO₂ assimilation. Consistent with this hypothesis is the observation that emergent I. bolanderi plants, resulting from fluctuating water levels, initiated leaves with stomata (unlike adjacent submerged plants) and, although these leaves had substantially higher chlorophyll levels, they showed an order of magnitude less acid fluctuation than submerged leaves.