A detailed analysis of the replication and transcription patterns of the X chromosomes of Drosophila miranda has been made using 3H-thymidine and 3H-uridine autoradiography. The purpose of this investigation has been to examine the relative duration of replication and hyperactivity of the different regions of the X2 element of this species. Results reveal that there is a considerable amount of heterogeneity among different segments of the X2 but not among the segments of X1 chromosome. Secondly, there is a clear site wise relationship between early replication and hyperactivity in X1 as well as X2. Thirdly, the X2 element appears to replicate relatively earlier than the X1 element in the male. - Our data do not support a spreading effect or polarization in the distribution of hyperactive segments. Furthermore, the X2 element appears relatively thinner than X1 in almost all nuclei from male larval glands. The significance of these results has been discussed in the light of the conservativeness of dosage compensation mechanism in a particular evolutionary line. © 1982 Springer-Verlag.