The authors of this paper report on a satellited large acrocentric chromosome showing abnormally lengthened short arms. This chromosome was found in a mongoloid child (GLD-Trisomy), her mother, and the dizygotic twin sister of the mother, both phenotypically normal. The chromosome abnormality may have originated from a duplication of the short arms or from insertion of a piece of another chromosome between the short arms and the satellites. Since the total length of this chromosome exceeds that of the other D’s, a pericentric inversion is improbable. Normal variability and structural heterozygosity coinciding with non-disjunction are discusse