1. The microsporogenesis in Isoetes japonica especially reduction division was studied, and is described in the present paper. TAKAMINE reported the somatic number of chromosomes to be 43-45 in Isoetes japonica and 22 in I. asiatica. EKSTRAND confirmed the haploid number of chromosomes as 11 in the reduction division of I. echinosporum. In the present case of I. japonica 33 gemini were counted at the metaphase plates during the reduction division. This seems to show that the plant used in the present investigation is hexaploid. 2. Chromatic threads run parallel two and two at leptonema and in paris in the synaptic stage. Though the double nature of chromatic spiremes can be discerned only with difficulty in pachynema stage, it becomes clear again at diplonema. Nucleoli can not be seen in the nucleus after the synaptic stage. 3. After the first meiotic division is normally concluded, the “Scheidewand” is completed between the newly formed nuclei. The moving away of the chromosomes from the equatorial plane towards the opposite poles was not taken place simultaneously in the second meiotic division. 4. Tapetum cells become clearly discernible at the time when the spore mother-cells are completed, becoming arranged in one layer in most of the places. Tapetum cells always remain intact, instead of being broken down as is usually the case in the ferns and Equisetums.