Sex chromosomes inOncopeltus fasciatusandPyrrhocoris apterusare heterochromatic and transcriptionally inactive in well-defined phases of development. InOncopeltus, which has the XX/XY type of sex determination, the Y-chromosome seems to be inactive in aE tissues. The X-chromosome is inactivated during the last spermatogonial division and remains so throughout spermatogenesis. The X-chromosome ofPyrrhocoris, which is of the XX/XO type of sex determination is inactive in the same stages as that ofOncopeltusand during an additional phase in early oogenesis. X inactivation in spermatogenesis is interpreted as a form of gross regulation of gene activity on the chromosomal level.