Cytogenetic preparations from oocytes remaining unfertilised after in vitro fertilisation revealed single chromatids (as opposed to whole chromosomes) in 4 out of 38 meiosis II metaphases. In one oocyte, a single chromatid was present in addition to the normal 23,X complement, and in three oocytes, two identical but separate chromatids replaced one whole chromosome within the complement. The data indicate that the chromatids have arisen as a result of premature division of the centromeres at meiosis I ("predivision"). None of the oocytes were hyperhaploid with an extra whole chromosome. These findings are at variance with the general belief that trisomy in man is largely a consequence of nondisjunction of whole bivalents at meiosis I and they suggest that predivision resulting in separate chromatids may be a significant mechanism for human trisomy.