Peroxidase isc.zymes of leaf blades were determined at the pH ranging from 5.0 to 7.0 for 240 mulberry varieties by the thin-layer gel isoelectric focusing. A total of 10 kinds of the isozyme bands was found in these varieties (Figs.4 and 5). The varieties were roughly classified into 5 types concerning the isozyme patterns by visual inspection, according to the existence or absence and the stainlng intensity of 6 anodal bands such as A-4, A-5, A-6, A-7, A-8 and A-9. Type I, II, III, IV and V were composed of 38, 46, 13, 2 and 1 per cent of the total number of the varieties used, respectively. Since the experiment was carried out using leaf blades with various leaf age(Fig.1), tree age and planting location, an additional experiment was performed concerning these points and it proved that they had no effect on isozyme variation (Figs.-2 and 3). Varietal afEinities were evaluated from principal component analysis in which the data concerning the staining intensity of 6 anodal bands, namely, A-1, A-5, A-6, A-7, A-8 and A-9, were used (Tables 1 and 2). Both of the 1st and the 2 nd principal components consisted of 59 per cent of the whole of the information. The scatter diagram of scores of these 2 principal components suggests that the varieties of the same type most closely resemble each other, that the varieties of Type II show closer afiinity with those of Types I and III, and that the varieties of Types lv and V show a rather low affinity with those of the other types (Fig.6). Most botanists agree that the majority of the 240 mulberry varieties examined are involved in 3 species, namely, Morus bombycis KOIDZ., M. alba L. and M. latifolia POIRET, depending on the difference in morphological characteristics. Within limits of the experiment, the relationship between those 3 species and the 5 types of isozyme patterns is not yet clarified.