Although C. elegans is commonly used as a model organism for studies of cell biology, development and physiology, the small size of the worm has impeded measurements of ion transport by the excretory cell and hypodermis. In this paper we have used the scanning ion-selective microelectrode technique to measure efflux and influx of K+, H+, Na+ and Ca2+ in intact worms. Transport of ion into, or out of, immobilized worms produces small gradients in ion concentration in the unstirred layer near the worm’s surface. These gradients are readily detectable with ion-selective microelectrodes and the corresponding ion fluxes can be estimated using the Fick equation. Our data show that effluxes of K+, H+, Na+ and Ca2+ are localized to the region of the excretory pore, consistent with release of these ions from the excretory cell, and that effluxes increase after experimental preloading with Na+, K+ or Ca2+. In addition, the hypodermis is a site of Na+ influx.