The paper describes genetic and cytological studies of mice carrying a sub-metacentric chromosome, apparently formed by the combination of a medium-sized acrocentric chromosome with a small one having a secondary constriction. The chromosome arose spontaneously in a stock of mice carrying Cattanach’s translocation and has been designated translocation T163H. Heterozygotes and homozygotes of both sexes have 39 and 38 chromosomes respectively and are phenotypically normal and fertile. The fertility of male heterozygotes is somewhat reduced, but females are fully fertile. There was a normal Mendelian segregation of the chromosome among the progeny. Cytologically, there was little evidence of instability of the chromosome, and at mitosis the long and short arm appeared equivalent in length to the tenth longest and the shortest acrocentric chromosomes respectively. At first meiotic metaphase in male heterozygotes a trivalent was recognised in all cells, and at second metaphase 98% of cells had a balanced chromosome complement with either 18 acrocentrics and a metacentric or 20 acrocentrics. Homozygous males had 19 bivalents at first meiotic metaphase, and that formed by the metacentrics was identifiable in many cells as a large ring bivalent. The translocation had a chromosome in common with translocation T138Ca, thought to be the chromosome carrying linkage group II. The chromosome provides a useful marker. The mechanism of its formation is not know