The human intestinal pathogen Giardia lamblia is a flagellated unicellular protozoan with pronounced medical and biological relevance. However, the basic physiology of Giardia trophozoites has been sparsely studied, especially the electrical and ionic properties of their cellular membrane which are virtually unknown. In this study, we were able to record and characterize the macroscopic ionic currents of Giardia trophozoite membrane by electrophysiological methods of the patch clamp technique. Giardia trophozoites showed a high current density (∼600 pA/pF at −140 mV) that was activated upon hyperpolarization. This current was carried by a chloride-selective channel (I _Cl-G) and it was the most important determinant of the membrane potential in Giardia trophozoites. Moreover, this conductance was able to carry other halide anions and the sequence of permeability was Br⁻ > Cl⁻ ≈ I⁻ ≫ F⁻. Besides the voltage-dependent inward-rectifying nature of I _Cl-G, its activation and deactivation kinetics were comparable to those observed in ClC-2 channels. Extracellular pH modified the voltage-dependent properties of I _Cl-G, shifting the activation curve from a V _1/2 = −79 ± 1 mV (pH 7.4) to −93 ± 2 mV (pH 8.4) and −112 ± 2 mV (pH 5.4). Furthermore, the maximal amplitude of I _Cl-G measured at −100 mV showed dependence to external pH in a bell-shaped fashion reported only for ClC-2 channels. Therefore, our results suggest that I _Cl-G possesses several functional properties similar to the mammalian ClC-2 channels.