Recently, Werner and Stelzer reported C3 gas exchange simultaneous with CAM acid fluctuatíons, i.e., CAM-cycling, in seedlings of the mangrove species Rhizophora mangle L. The potential for CAM-cycling was investigated in mature individuals of R. mangle and of Laguncularia racemosa (L.) Gaertn. f in šitu in southwestem Florida and in seedlings of R. mangle grown in a growth chamber. Only C3 CO2 exchange was observed, while diumal fluctuatíons of tissue acid content were absent. Thus, no evidence for CAM-cycling was found in the field or in the growth chamber for R. mangle, nor in the field for L. racemosa.