In order to determine if a mitotic spindle organizing center is present in dinoflagellate cells, we used a library of 12 monoclonal antibodies obtained by immunizing mice with isolated human centrosomes. When tested by immunofluorescence on cryosections of the dinoflaggelate Crypthecodinium cohnii B., a positive labeling was obtained with three of these antibodies. In interphase cells, the anti-centrosome antibodies labeled structures located either in the cell periphery, corresponding probably to both basal bodies (i.e. kinetosomes) and in the perinuclear area. In the latter case, two punctate structures were observed near the nuclear envelope. They have never been described, either in light, or in electron microscopic studies of dinoflagellates. We have designated them as centrosome-like structures. A microtubular desmose reacting positively with anti-tubulin Ab was also visible, linking kinetosomes and centrosome-like structures. During mitosis, the double punctate structures were observed at the poles of the nucleus. Double immunolabeling with tubulin and anti-centrosome Ab was also carried out and strongly suggested that in mitotic cells, centrosome-like structures, located at the poles of the mitotic spindle, were associated with microtubular bundles and probably organize and polarize them. These data indicate the existence of centrosome-like structures in C. cohnii cells and the strong conservation of some centrosomal epitopes from dinoflagellates to human. One of the antibodies (CTR 210) recognized by immunoblotting, a single protein band at 72 kDa from a total protein extract. The direct demonstration that this protein is located at the centrosome-like structures and at the kinetosomes deserves further study.