InGryllotalpa fossor (Orthoptera) (23, X0 male; 24, XX female) we have established the existence of random X chromosome inactivation for dosage compensation of X-linked genes. Both cytogenetical (DNA replication and transcription) and biochemical (X-linked glucose-6-phosphate dehydrogenase) studies have indicated that one of the two X chromosomes in the female soma (hepatic caeca) is late replicating and transcriptionally silent leaving the other X chromosome to remain active as in males thereby ensuring the production of almost the same amount of X-linked glucose-6-phosphate dehydrogenase in both sexes. Even in oogonia, one of the two X chromosomes continues to retain inactive. Only prior to their entry into meiosis the inactive X chromosome is reactivated. Accordingly, there is two-fold increase m the level of X-linked glucose-6-phosphate dehydrogenase in oocytes, From this it is implied that the restoration of X chromosome inactivation should occur some time during early embryogenesis. Thus, dosage compensation inGryllotalpa seems to be analogous to that in mammals. Our work bears testimony to the ancient origin of this mechanism.