A microelectrode is described which permits a nondestructive method for measuring (\rho \mathrm{CO}{2}) in living cells or small samples. Our objective was to develop an easily fabricated (\rho \mathrm{CO}{2}) microelectrode that would permit analysis of small samples and withstand insertion into the soil. We developed a (\rho \mathrm{CO}{2}) microelectrode by making use of a newly developed micro-pH electrode (MEPH4, WPI) and a semiliquid chlorotrifluoroethylene oil-wax gas permeable membrane. The fabricated microelectrode has a tip diameter of (\leqslant 120 \mu \mathrm{m}) (o.d.) and a response time of (\leqslant 3 \mathrm{~min}) with a linear (nearly Nernstian) slope (\left(58.0 \pm 2 \mathrm{mV} / \log {10}\right.) (\rho \mathrm{CO}{2}) ). The (\mathrm{CCO}{2}) microelectrode response is independent of the test solution (\mathrm{pH}) and electrolytes. Measurements of (\rho \mathrm{CO}{2}) with the microelectrode compared favorably with those from a standard (\rho \mathrm{CO}{2}) macroelectrode (model 95-02. Orion), and the regression line had a slope of 1.2 and (r=0.98). This microelectrode is simple to fabricate, and the tip diameter is sufficiently small ((\leqslant 120 \mu \mathrm{m})) to permit measurement of (\mathrm{CO}{2}) in small sample volumes. The first direct potentiometric determinations of rhizosphere (soil-root interface) (\mathrm{pCO}{2}) were carried out to demonstrate the performance of the microelectrode. Rhizoplane ((<1 \mathrm{~mm}) from the root) (\rho \mathrm{CO}{2}) values at the five leaf stage were higher for Sordan-757 (9.14 (\pm 0.39) and (11.33 \pm 0.28 \mathrm{kPa})) than for Sordan-333 ((7.15 \pm 0.57) and (9.55 \pm 0.30 \mathrm{kPa})) on low and high (\mathrm{CaCO}{3}) soils, respectively. The (\rho \mathrm{CO}_{2}) microelectrode is an improved technique for examining the root-soil interface and other biological microenvironments. E 1993 Academic Press, Inc