ABSTRACT An investigation of extracellular ionic currents in Acetabularia acetabulum is presented. Utilising the vibrating electrode technique it is shown that there are large (up to 380μAcm-2) light-dependent and small (up to lOμAcm-2) light-independent extracellular ionic currents around Acetabularia. The current density was greatest at the rhizoid and diminished towards the developing apex where there was no measurable current. Ion-replacement and ion-transport inhibitor studies indicated that Cl-was the main constituent of the light-dependent and light-independent currents. Calcium ions were found to contribute about 5–15% of the light-independent current and <1% of the light-dependent current, as deduced from cobalt inhibition studies. Possible mechanisms responsible for these phenomena are discussed briefly.