Carbon and hydrogen isotope ratios of cellulose nitrate and oxygen isotope ratios of cellulose from species of greenhouse plants having different photosynthetic modes were determined. When hydrogen isotope ratios are plotted against carbon isotope ratios, four clusters of points are discernible, each representing different photosynthetic modes: C3 plants, C4 plants, CAM plants, and C3 plants that can shift to CAM or show the phenomenon referred to as CAM-cycling. The combination of oxygen and carbon isotope ratios does not distinguish among the different photosynthetic modes. Analysis of the carbon and hydrogen isotope ratios of cellulose nitrate should prove useful for screening different photosynthetic modes in field specimens that grew near one another. This method will be particularly useful for detection of plants which show CAMcycling.There are three major photosynthetic pathways in plants: C3,2 C4, and CAM. Carbohydrates in all three are synthesized by the Calvin cycle. In C3 plants, CO2 is fixed directly into the Calvin cycle, with the primary carboxylation product being a threecarbon compound (11). In C4 and CAM plants, the initial carboxylation product is a four-carbon compound (10,