Chromosomes from embryos (stage 34) of the amphibian urodele Pleurodeles waltlii were labeled by 3H-thymidine at different times of the DNA synthesis cycle and studied at metaphase. Terminal segments of the chromosomes, induced secondary constrictions, and satellites were found to be late-labeling. Therefore, they may be considered as formed at least partially by heterochromatin. However, after other treatments or stainings, they do not present other features considered characteristic of heterochromatin. The question of the heterogeneity of their structures is discussed and the ambiguity of the term heterochromatin is emphasized. The proximal part of the long arm of chromosome VII shows characteristics similar to satellites, which may indicate the presence of a previously unrecognized satellite.