Metaphase chromosomes from three reciprocal translocation carriers were examined by scanning electron microscopy. A gap was detected in one of the two derivative chromosomes in each carrier. The position of each gap was identical with that of the respective breakpoint of the derivative chromosome to which the longer chromosome fragment was translocated. The gap length resembled a midsize G-band. Our observations suggest that derivative chromosomes of reciprocal translocations involve aberrations of a high-ordered structure of chromatid fibers at the translocation sites.