The unhatched, newly hatched and free-swimming coracidia of Bothriocephalus clavibothrium Ariola, 1899 an intestinal parasite of the teleostean fish Arnoglossus laterna (Walbaum, 1792) (Pleuronectidae) from the Mediterranean Sea near Sete, France were examined by means of transmission electron microscopy (TEM) and cytochemistry methods. Each unhatched and hatched coracidium of B. clavibothrium is composed of a hexacanth larva, oncosphere, about 28 µm in diameter, surrounded by a 5-7 µm thick ciliated envelope containing about 16 mesomere nuclei and a large amount of nutritive reserves (alpha- and beta-glycogen and lipids). Several cell types were distinguished within the oncosphere: (1) binucleated subtegumental cell; (2) two flame cells; (3) binucleated penetration gland; (4) two nerve cells; (5) about 100 intermediate, somatic cells which represent perikarya of both somatic and hook musculature; (6) about 50 small embryonic cells, some of them with pycnotic nuclei containing very dense chromatin and showing evident signs of regression and degeneration; and (7) about 10 to 12 germinative cells with prominent nucleoli in large lobate nuclei, surrounded by a thin layer of compact, granular cytoplasm rich in RNA. Apoptosis of numerous micromeres and of outer-envelope and ciliated-envelope material was frequently observed. The total number of oncospheral cells is about 160, the highest number reported from various orders and families of cestodes. This number, along with our own data from different cestode orders and families, supports our hypothesis that the progressive reduction in oncosphere cell numbers is correlated with a simplification of the infective larval stages and is a general trend in cestode evolution. This correlation represents an interesting ontogenic adaptation to the parasitic way of life.