The structure of the kinetosomes of zoospores of the Babraham isolate of Neocallimastix (N. patriciarum, previously described as N. frontalis) both free and in the later stages of zoosporogenesis have been determined from electron micrographs of thin sections. The zoospores have up to 20 flagella. At the base of each, within a circular ridge of the plasma membrane, is a circumflagellar ring. There are no props connecting the triplets of the kinetosome to the plasma membrane, but there are two connections from the skirt around the kinetosome to the circumflagellar ring. In developing zoospores, within sporangia, the circumflagellar ring is perpendicular to the long axis of the flagellum, but in free zoospores it is tilted at an angle of about 60°. The skirt is continued proximally to form a cowl-shaped structure below the kinetosome. The spur has longitudinal and transverse components; microtubules radiate from both. Some microtubules intersect the nucleus, but there is no specific association. The classification of Neocallimastix is discussed.