Short-term discrimination in assimilation of stable isotopes of carbon was measured for leaves of the C3 speciesPhaseolus vulgaris L. cv. Hawkesbury Wonder andFlaveria pringlei Gandoger, the C4 speciesAmaranthus edulis Speg., and the C3-C4 intermediate speciesPanicum milioides Nees ex. Trin,Flaveria floridana Johnson, andFlaveria anomala B.L. Robinson. Discriminations in the C3 and C4 species were similar to those expected from theoretical considerations. When ambient CO2 pressure was 330 μbar the mean discriminations in the C3 species andPanicum milioides were similar, whereas the mean discriminations inF. floridana andF. anomala were less than discrimination in C3 species andPanicum milioides. When ambient CO2 pressure was 100 μbar the mean discriminations inPanicum milioides andF. anomala were greater, and that inF. floridana was less, than that inPhaseolus vulgaris. We conclude that the pattern of discrimination inPanicum milioides is consistent with the presence of a glycine shuttle; inF. floridana andF. anomala, discrimination is consistent with the presence of a C4 pathway coupled with the operation of a glycine shuttle.