Reciprocal translocations are readily induced by various physical and chemical mutagens in certain germ-cell stages. Carriers of balanced reciprocal translocations generally exhibit no abnormal phenotypes, except for occasional male sterility, but about half (on average) of their progeny carry grossly unbalanced chromosome complements and die prenatally, so that the carriers are said to be ‘semisterile’. Since death of such progeny generally occurs in very early embryonic stages, it would be of minor importance in an analogous human situation. Several types of unbalanced segregants, however, survive to late gestational or even to postnatal stages and are often malformed. Recently, it was determined in this laboratory that over one half of the male carriers of methylene-bisacrylamide-induced translocations, sired litters that had late-dying and/or malformed fetuses (Rutledge et al, 1990). Five high-anomaly translocation stocks derived from that study and four derived from studies with other mutagens were analyzed cytogenetically to determine (1) the chromosomes and breakpoints involved, (2) the nature of chromosome imbalance in malformed fetuses, and (3) the types of meiotic segregation that produce late-surviving unbalanced segregants. Cytogenetic analysis of the 9 translocation stocks revealed 18 breakpoints located in 12 chromosomes. Each translocation had at least one breakpoint located near the centromere or the telomere. All translocations produced abnormal fetuses that were partially monosomic for a very short terminal chromosome segment, and partially trisomic for a segment that can be of various lengths, 2–10 times as long as the monosomic segment. In 6 stocks, these abnormal fetuses arose by adjacent-1 or alternate segregation; in the other three they arose by adjacent-2 segregation. In addition, tertiary trisomy by 3–1 missegregation was also observed in two of the stocks.