1. In short-term tissue culture, bone marrow cells of adult chicken were treated with tritiated thymidine for the last 2–4 hours prior to fixation. Autoradiographs made from these cells permit the study of the final stages of DNA replication in the chromosomes. 2. Uptake of tritiated thymidine was found in both macro- and microchromosomes. Generally, the uptake ceases earlier in the microchromosomes than it does in segments of all six pairs of macrochromosomes. In the macrochromosomes, DNA synthesis is completed according to a distinct pattern. 3. In the homogametic male sex, the two Z-chromosomes were found to replicate DNA and to cease replication at a rate and time comparable to the other macrochromosomes. There is no significant difference in labeling pattern between the two Z’s. This is a remarkable contrast to the situation found in the homogametic female sex of the mammals. 4. In the heterogametic female birds, the single Z-chromosome showed a replication pattern not different in rate and relative timing from the two Z-chromosomes of the male. 5. In all of the labeled female cells and in none of the labeled male cells, a single element of the larger microchromosomes exhibited a very heavy and comparatively late uptake of tritiated thymidine. This observation is strongly reminiscent of the replication pattern of the mammalian Y-chromosome. It appears that this late labeling microchromosome is identical with the W-chromosome first reported by Frederic (1961).