An unusually long secondary constriction on chromosome 9 was shown by C-staining to involve duplication of heterochromatin in this region. The constriction stained darkly following the C-staining procedure but was unstained following the G-staining procedure. The heterochromatic constriction generally completed DNA replication earlier than the remainder of chromosome 9. The abnormal chromosome was detected in 17 individuals belonging to three generations of a family. There were twice as many carriers as noncarriers. Similar excesses of carriers were noted in two other pedigrees reported in the literature. Disturbed segregation of this abnormal chromosome 9 is suggested and is consistent with the increased centromeric heterochromatin present. No linkage to chromosome 9 was demonstrated, but Rh-linkage was discounted.