Gas exchange of Eurasian watermilfoil (Myriophyllum spicatum L.) indicated a near-zero CO(2) compensation point and a high temperature optimum for photosynthesis. These properties are characteristic of plants fixing CO(2) by a beta-carboxylation mechanism. Operation of the Calvin cycle was shown and no evidence for beta-carboxylation was obtained. These results indicate that near-zero CO(2) compensation points are not dependent on a beta-carboxylation mechanism.