Giardia intestinalis is a primitive microaerophilic protozoan parasite which colonises the upper intestine of humans. Despite the evolutionary and medical significance of this organism, its physiology is very poorly understood. In this study we have used a novel flow cytometric technique to make quantitative measurements of the electrical potential across the plasma membrane of G. intestinalis trophozoites. In media lacking both K(+) and Na(+), G. intestinalis trophozoites maintained a high negative plasma membrane potential (Psi(m)) of -134+/-3 mV. The Psi(m) was unaffected by the addition of Na(+) to the extracellular medium, whereas the addition of K(+) resulted in a significant membrane depolarisation, consistent with the G. intestinalis trophozoite plasma membrane having a significant (electrophoretic) permeability to K(+). The membrane was also depolarised by the H(+) ionophore m-chlorophenylhydrazone and by the H(+) ATPase inhibitors dicyclohexylcarbodiimide and N-ethylmaleimide. These results are consistent with G. intestinalis trophozoites maintaining a high resting Psi(m), originating at least in part from an electrogenic H(+) pump acting in concert with a K(+) diffusion pathway.