Pachytene preparations of the chromosome complement of female larvae of Bombyx mori were improved to give a distinct chromomere pattern of the bivalents suitable for chromosome mapping. Six of the 28 bivalents are described and can be identified regularly in the bivalent complements, among them the bivalent containing the nucleolus organizer. The relative lengths of these bivalents compared with one another change during development of pachytene. In contrast to other members of the Lepidoptera there is no conspicuous heterochromatic W-chromosome, which corresponds to the female-specific heterochromatin body present in the nuclei of somatic tissues. This tissue-specific heteropycnosis indicates a different functional state of the responsible chromosome or chromosomal segment in germ line and somatic cells.