Cellulose from plants having crassulacean acid metabolism was enriched in deuterium but not in oxygen-18 in relation to cellulose from C3 and C4 plants growing in the same area, indicating that the deuterium enrichment is due to isotopic fractionation during biochemical reactions rather than during evapotranspiration. Hydrogen and oxygen stable isotope ratios of cellulose from the plants in this restricted area showed more variability than that observed in samples collected across an entire continent. Biological factors appear to be as important as environmental factors in determining the isotope ratios of plant cellulose.