Plane-parallel ionization chambers that exhibit polar effects with low energy electron beams are recommended for therapeutic electron dosimetry. In this study, the polarity effects of a C-134A ionization chamber, a major commercially available plane-parallel ionization chamber in Japan, were characterized as a function of mean energy at various depths. Polarity effects were measured at representative depths along depth dose curves of nominal 4, 6, 9, 12 and 15 MeV electron beams, and were compared with previously reported results. Polarity errors for the ionization chamber studied were shown to monotonically increase with decreasing mean energy at a given depth and were maximal at about 1-2 MeV. It was also shown that polarity errors depended on the energy of the incident electron beam. The polarity error of the C-134A ionization chamber was larger than that of other previously investigated plane-parallel ionization chambers. Because the magnitude of polarity effects should be determined throughout the depth dose curve in therapeutic electron dosimetry, it is always necessary to measure ionization readings taken at both polarities.