The hyphae of filamentous fungi exhibit a highly polarized mode of cell extension and all generate electrical currents around their hyphal tips. Of the 13 fungi that have been examined with the vibrating probe, 12 normally have positive electrical current entering the growing tip. However, the apical current is always outward in Allomyces macrogynus. In those organisms that have inward apical currents, outward currents are found occassionally and transiently in hyphae that extend at normal rates. There is no good correlation between either the extension rate or the length of the growing peripheral cell and the current density. These studies lead one to conclude that the relationship between the generation of an electrical current and the extension of a hypha is superficial and not essential. The association between the flow of protons, which carry most of the current in fungi, and cell extension may be tighter. Comparison of the currents of three water molds A. macrogynus, Blastocladiella emersonii and Achlya bisexualis provides an insight to the probable role of the electrical current. A. macrogynus and B. emersonii both have rhizoids that may take up nutrients by proton-linked transport. The nutrients support growth of the sporangium and hypha, respectively, to which they are joined. In A. bisexualis, proton symport is responsible for nutrient transport into the hyphal tip. There seems to be an excellent correlation between the zones at which proton-coupled nutrient transport occurs and the regions where positive current enters these fungi. Therefore, the current may indicate local nutrient uptake rather than local cell growth.