Summary—The major basic nuclear protein HCc (previously named Histone‐like) of the dinoflagellateCrypthecodinium cohniiB was immunolocalized in light and electron microscopy using an affinity‐purified polyclonal antibody. Complementary conventional and cryo‐techniques were used to study the distribution of the DNA‐binding protein in interphase cells and to follow its behaviour throughout the mitotic cycle. In non‐dividing cells, the HCc protein was found to be located on extra‐chromosomal loops and chromosomal nucleofilaments dispersed in the nucleoplasm. In mitotic cells, from prophase to early telophase, it was homogeneously distributed in the (whole) dividing chromosomes. HCc protein was also detected in two compartments of all the permanently observable nucleoli: the nucleolar organizing region and the fibrillo‐granular region. In this paper we discuss the hypothetical roles, structural and/or functional, of this DNA‐binding protein, which is specific to dinoflagellates, the only eukaryotes whose chromatin is devoid of histone