Abstract In the silkmoth, Antheraea polyphemus, a region of the larval silk gland consists of cuticle-producing cells which, without dividing, ‘transdifferentiate’ into salt-secreting cells during metamorphosis to the adult moth. This functional transformation is accompanied by marked ultrastructural changes. In the normal course of events, elaboration of two mitochondria-rich zones of cytoplasm characteristic of the salt-secreting state starts shortly after the beginning of a period of DNA synthesis. Nevertheless, this correlation may not reflect a causal relationship. By the use of mitomycin C, a substantial suppression of the thymidine incorporation can be achieved, without interference with the transformation of the cells.