A division arrest mutant, cdaA, of Tetrahymena thermophila is known to have a ts-defect in the formation of the fission zone which determines the position of the fission plane. A protein (Mr = 85,000; pI = 4.7, designated as p85) has recently been identified in our laboratory as a possible gene product of the cdaA locus by two-dimensional gel electrophoresis and genetic experiments (Ohba et al., submitted). In the present research, we have isolated p85, prepared its antibody, and demonstrated that in wild-type cells or in cdaA cells at permissive temperature, immunofluorescence for p85 appears on the equatorial basal bodies at the predicted fission zone just before formation of the zone. In such a case, the fission zone appears to be formed just anterior to the fluorescence-associated basal bodies, and then constriction of the division furrow occurs at the zone. However, in cdaA cells at the restrictive temperature, the equatorial p85 deposit and subsequent fission zone formation and furrowing do not occur at all. Thus, we conclude that p85 plays a key role in the formation of the fission zone and in the positioning of the equatorial fission line.