Measurements of the resistance in series with the excitable membrane for giant axons of two different phylla (the squid Loligo pealii and the marine worm Myxicola infundibulum) were obtained. Efforts were made to take into account the errors introduced by the finite rise-time of the measuring apparatus. The series resistance value, obtained very quickly by the method described, may be used in setting the compensation potentiometer to offset this resistance in voltage-clamp measurements. Estimates of the resistance of the periaxonal tissue layer were made. Analyses were done on some of the problems involved in attempting to make an unambiguous determination of the series resistance.