Somatic interphase nuclei of different tissues of heterogametic females of various species of snakes with a differentiated W chromosome possess a distinct, darkly stained chromocenter comparable to the characteristic mammalian sex chromatin. The absence of such a prominent body in similar tissues of homogametic males, the allocycly in the DNA replication pattern exhibited by the W, and the positive correlation between the size of the chromocenter and the size of the W in different species are the evidence from which it is concluded that the sex-chromatin body of the interphase nuclei in snakes is formed by the W chromosome of the heterogametic females. The lack of incorporation of 3H-uridine by the chromocenter indicates its inactivity. In order to distinguish this heteropycnotic body from the sex chromatin of the homogametic sex in mammals, it is suggested that it be termed “W chromatin