Electron microscopic examination of serial sections of developmental stages of the flagellar apparatus during the cell cycle indicates that the basal bodies replicate in a semi-conservative manner and that there is a flagellar transformation over two cell cycles in euglenoids as in other algal flagellate groups. Two new pairs of basal bodies are formed, each pair comprising one parental and one newly developed basal body. There is a transformation of the parental dorsal flagellum containing a thin paraxonemal rod into a ventral flagellum bearing a large paraxonemal rod. Observation of the roots associated with the basal bodies shows that the dorsal root transforms into an intermediate root over two cell cycles following the transformation of the dorsal basal body/flagellum to a ventral one. Also the two ventral roots are newly formed in relation to the formation of two new phagotrophic apparatuses during the division. After the breakage of the connection between the parental basal bodies the two new pairs move apart and are guided/drawn by transverse microfibrillar bundles which connect them to opposite sides of the pellicle. The axis of the separation/migration of the pairs of basal bodies is parallel to the axis of elongation of the dividing nucleus.