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During oncospheral differentiation in Anoplocephaloides dentata the degeneration of numerous micromeres, transformed in dense, pycnotic nuclei was observed. It results in an evident decrease in the number of oncospheral cells, from about 70 observed in the early preoncospheral stage to about 26 remaining in the infective oncospheres. The mature oncosphere of A. dentata consists of three pairs of oncospheral hooks, an extensive peripheral, somatic musculature interconnected with a complex system of hook muscles and five cell types: nerve cells, syncytial penetration gland, oncospheral tegument complex, as well as small somatic and large germinative cells. The approximate number of oncospheral cells in A. dentata is 26 and number of oncospheral nuclei is 28. This number includes two syncytial structures, a bi-nucleate tegumental perikaryon and a bi-nucleate penetration gland, as well as about 17 somatic cells which represent perikarya of somatic and hook musculature, 2 nerve cells and 6 germinative cells. The results of this study provide supplementary information and support our hypothesis that the progressive reduction in the number of oncospheral cells is a general trend in cestode evolution (Swiderski 1983) and may represent one of their ontogenetic adaptations.