We provide evidence for an unusual behavior of the cyclin B homologue, p56, in the dinoflagellate Crypthecodinium cohnii. p56, of which we previously demonstrated the presence in this original eukaryotic protist, is present all along the cell cycle progression, and is exclusively cytoplasmic as revealed after immunofluorescence labeling with anti-p56 Ab and counterstaining with Dapi. It was never found in the nucleus as is the case in higher eukaryotic cells. During mitosis, p56 was essentially associated with the mitotic apparatus: centrosomes and mitotic spindle, as shown after double immunofluorescence labeling with anti p56 and anti beta-tubulin Ab. Using high pressure freeze fixation, we clearly detected in transmission electron microscopy (TEM) the localization of p56 cyclin B homologue and beta-tubulin: single immunogold labeling demonstrated that p56 is localized along the whole cell cortex, along the cleavage furrow of anaphase to cytokinesis cells and into cytoplasmic channels passing throughout the mitotic nucleus where is located the mitotic spindle. Double immunogold labeling realized with anti-p56 and anti-beta-tubulin antibodies confirm that p56 antigens colocalize with beta-tubulin in many sites. The significance of the exclusively cytoplasmic localization of the cyclin B homologue is discussed.