The assimilation of laCO2 into the Ca acids malate and aspartate by leaves of C3, Ca and C3-Ca intermediate Flaveria species was investigated near the CO2 compensation concentration (F*) in order to determine the potential role of phosphoenolpyruvate (PEP) carboxylase (EC 4.1.1.31) in reducing photorespiration in the intermediates. Relative to air concentrations of CO2, the proportion of CO2 fixed by PEP carboxylase at F* increased in all six C3 Ca intermediate species examined. However, F.floridana J.R. Johnston and F. ramosissima Klatt were shown to be markedly less responsive to reduced external CO2, with only about a 1.6-fold enhancement of CO2 assimilation by PEP carboxylase, as compared to a 3.0-to 3.7-fold increase for the other C3 Ca species examined, namely, F. linearis Lag., F. anomala B.L. Robinson, F. chloraefolia A. Gray and F. pubescens Rydb. The Ca species F. pringlei Gandoger and F. cronquistii A.M. Powell exhibited a 1.5-and 2.9-fold increase in labeled malate and aspartate, respectively, at F*. Assimilation of CO2 by PEP carboxylase in the Ca species F. trinervia (Spreng.) C. Mohr, F. australasica Hook., and the C4-1ike species F. brownii A.M. Powell was relatively insensitive to subatmospheric levels of CO2. The interspecific variation among the intermediate Flaverias may signify that F. floridana and F. ramosissima possess a more Ca-like compartmentation of PEP carboxylase and ribulose-l,5-bisphosphate carboxylase/oxygenase (EC 4.1.1.39) between the mesophyll and bundle-sheath cells. Chasing recently la-*Published as Paper No. 8832, Journal Series, Nebraska Agricultural Research Division