DNA ploidy and mitotic activity were microspectrophotometrically investigated in 46 patients with gastric carcinoma. Measurements of D N A content and mitotic index (M.I.) were examined in the mucosal, submucosal, muscularis propria, and serosal layers of tumors, respectively. The frequency of cells with values exceeding hexaploid chromosome (6c) and mitotic counting analysis revealed a significant higher value in serosa, compared with findings in the mucosa. This tendency was not evident in cases of a differentiated type adenocarcinoma but was noted in those with the undifferentiated type. There were 37 tumors (80.0%) with the same DNA distribution patterns in every layer of the stomach (homogeneous DNA ploidy). Heterogeneity of DNA ploidy was observed in nine tumors (20.0%). Carcinoma with a heterogeneous D N A ploidy manifested a significantly higher incidence of metastasis to the lymph nodes than did those with homogeneous type. Characteristically, there was venous permeation preponderance in the differentiated type and peritoneal dissemination preponderance in the undifferentiated type. This evidence of DNA heterogeneity in gastric carcinoma tissues suggests a possible correlation with the metastatic behavior.