When subjected to low temperature, certain segments of the chromosomes of Triton vulgaris, T. palmatus and T. cristatus are heterochromatic. At mitosis these segments are undercharged and at meiosis uncharged with nucleic acid. These segments show the same type of allocyclic behaviour as do similar segments in Paris, Trillium and Fritillaria. They form Feulgen-positive chromocentres in all diffuse nuclei except pachytene, which is diffuse in Triton. The availability of nucleic acid at the stage when the chromosomes normally spiralize is now shown to be a condition of that spiralization. The diffuse pachytene without chromocentres is followed by meiosis with unspiralized heterochromatin: the diffuse resting nucleus with chromocenters is followed by mitosis with spiralized heterochromatin. With certain exceptions heterochromatin seems to be confined to those parts of chromosomes where chiasmata and crossing-over rarely occur.