The term “black children” in Chinese refers specifically to children who do not have a hukou, that is, those who are not officially registered in the national household registration system. They lack legitimacy and basic rights, such as access to medical care, education, and even employment. Living in marginalization of the society, they suffer from adverse well being. In order to enhance our limited understanding of this group of overlooked children, this study, using Chinese 1990 census 1% sample data, examines the determinants of un-registration of infants under 18 months old in China. It is estimated that un-registration can be caused by out-of-plan births, migration status, or simply malpractice of the local registration administration. In examining sets of child, maternal and community characteristics, it is identified that the migration status of the mother, and residential type of the community (rural or urban) are the key determinants of infant registration status. The results have policy implications for reforming the household registration system and improving child well being.