Some characteristics of peroxisomes in the slime mold, Dictyostelium discoideum were studied biochemically. The slime mold contained only two peroxisomal enzymes, urate oxidase and catalase. Both activities were concentrated highest in the light mitochondrial fraction, while the highest activity of acid phosphatase as a marker of lysosomes was found in the heavy mitochondrial fraction. Sucrose density gradient centrifugation showed that the density of peroxisomes was 1.21--1.22 g/ml, and that of lysosomes was approximately 1.21 g/ml. When the light mitochondrial fraction was treated with deoxycholate, major activities of urate oxidase, catalase, and acid phosphatase were solubilized. With development of the slime mold, both activities of urate oxidase and catalse increased at the stationary stage (4--6 h after) to a great extent and also slightly at the aggregation stage (10--12 h), thereafter they decreased gradually. Acid phosphatase remained with moderately high activity till the culmination stage (18--20 h), and then disappeared rapidly. It was found that enhanced activities of peroxisomal enzymes at the stationary stage represented an increase in the particle fraction and supernatant. On the other hand, activities of acid phosphatase at the same stage increased two-fold only in the supernatant, while the activity in the particle fraction was reduced to one-half.