The fine structure of microgametogenesis and fertilization in Leucocytozoon caulleryi was studied. When the peripheral blood of chickens containing mature gametocytes was withdrawn or ingested by Culicoides arakawae , differentiation of the microgametocytes began immediately. The cytoplasm of host cell and the outer membrane of the microgametocyte disappeared. Axoneme formation began in the cytoplasm and the condensed chromatin masses were observed at the periphery of the nucleus of microgametocyte after disappearance of the nuclear membrane. The axonemes and chromatin masses migrated to the surface of the microgametocyte and participated in the formation of flagellar buds. The flagellar buds passed through interruptions of the inner membrane and were surrounded by the outer membrane of the microgametocyte . The microgametocyte formed eight slender microgametes. The microgamete was a long undulated thread-like structure and composed of a single axoneme and a nucleus. The nucleus was located at the mid to anterior region of the microgamete . The axoneme consisted of nine peripheral doublets arranged around two central microtubules. Penetration of the macrogamete by the microgamete occurred rapidly after ingestion. The axoneme and nucleus of the microgamete were seen free in the cytoplasm and nucleus of a fertilized macrogamete.