The ultrastructural changes occurring during microgametogony in Tyzzerio parvula were studied in tissue from the small intestine of infected young domestic geese. The development of the trophozoite into the microgametocyte was accompanied by cytoplasmic growth and nuclear division. The developing microgametocyte could be differentiated from developing schizont because the nuclei of the microgametocyte were smaller than those of the schizont and lacked nucleoli. Every nucleus migrated to the edge of the microgametocyte accompanied by the aggregating of chromatin at the periphery of the nuclei. Then each nucleus became elongated, protruding outwards from the microgametocytic membrane, and developed a dense osmiophilic area which contained the condensed chromatin, and a pale area. The dense portion of the nucleus, accompanied by the mitochondrion, entered the protrusion to become the nucleus of the microgamete, and the pale portion remained in the residual body. The two centrioles above the ucleus transformed into the basal bodies from which the flagella arose. The microgamete consisted of two flagella, an elongated nucleus, a mitochondrion located at the anterior end, and microtubules alongsaide.