A 60 mm crown-rump length anatomically normal male fetus is described in which approximately 7 to 19% of the somatic and extraembryonic cells are sex chromatin positive. About an equal percentage of cells grown in culture from various regions of the body were also sex chromatin positive and chromosome analysis of replicate cultures showed that a proportionate number of cells had a total of 47 chromosomes with XXY sex chromosomes. The remaining cells had a normal male chromosome complement. Single sex chromatin positive nuclei or groups of nuclei, sometimes numbering several hundred, were found in the tissues derived from all primary germ layers throughout the entire body and extraembryonic membranes. On the basis of these findings it is suggested that this mosaic condition arose at a very early stage, possibly at the third somatic division, but certainly before tissue differentiation had occurred, through a non-disjunction of a normal male cell. This study sheds some light on the mechanics of embryonic development. The value of the sex chromatin as a marker for detecting mosaic conditions involving the X chromosomes is well illustrated as is the fact that a mosaic can be overlooked when samples are taken from a limited region of the body or if only a small number of cells are studied.