Representative mycelial fungi from the phycomycete, ascomycete and basidiomycete groups (Achlya bisexualis, Neurospora crassa, Aspergillus nidulans, Schizophyllum commune and Coprinus cinereus) all generated steady electrical currents around their hyphal tips; the generation of a transhyphal ion current may therefore be a universal characteristic of hyphal growth. As with all other tip growing organisms, positive current always entered apically and left distally; non-growing hyphae did not drive transcellular currents. The current density, measured approximately 30 micron from the membrane surface at the hyphal tips, varied between 0.05 and 0.06 microA cm-2 in different fungi and tended to be larger in wider, rapidly extending hyphae than in thinner, slow growing hyphae. The possibility that these currents serve to localize growth at the apex is discussed.