The level of 2-carboxyarabinitol 1-phosphate (CA 1P) in leaves of 12 species was determined by an isotope dilution assay. C-14-labeled standard was synthesized from [2-C-14]carboxyarabinitol 1,5-bisphosphate using acid phosphatase, and was added at the initial point of leaf extraction. Leaf CA 1P was purified and its specific activity determined. CA 1P was found in dark-treated leaves of all species examined, including spinach (Spinacea oleracea), wheat (Triticum aestivum), Arabidopsis thaliana, and maize (Zea mays). The highest amounts were found in bean (Phaseolus vulgaris) and petunia (Petunia hybrida), which had 1.5 to 1.8 moles CA 1P per mole ribulose 1,5-bisphosphate carboxylase catalytic sites. Most species had intermediate amounts of CA 1P (0.2 to 0.8 mole CA 1P per mole catalytic sites). Such intermediate to high levels of CA 1P support the hypothesis that CA 1P functions in many species as a light-dependent regulator of ribulose 1,5-bisphosphate carboxylase activity and whole leaf photosynthetic CO2 assimilation. However, CA 1P levels in spinach, wheat, and A. thaliana were particularly low (less than 0.09 mole CA 1P per mole catalytic sites). In such species, CA 1P does not likely have a significant role in regulating ribulose 1,5-bisphosphate carboxylase activity, but could have a different physiological role.