RANDOM X inactivation makes the female mammal a natural mosaic for clones of cells having either the maternally derived X (Xm) or paternally derived one (Xp) which is genetically inactive1. There are, however, instances in which inactivation is obviously not random2–7. Non-randomness was inferred from studies made on differentiated cells remote from early embryonic cells in which inactivation occurred. Thus it is not clear whether the randomness of the X inactivation process was influenced or whether cell selection occurring after random inactivation was responsible for the ultimate non-random appearance4–9. In an effort to determine the embryonic stage at which the X chromosome initiates differentiation in famale mouse embryos heterozygous for Cattanach's translocaton10, we found that the mosaic composition was consistently biased in extraembryonic membranes, whereas it was not necessarily so in the embryonic body.