Summary. Fewer frozen–thawed mouse oocytes cleaved to the 2-cell stage compared to fresh control oocytes fertilized in vitro (46% vs 79%). The reduced rate of 2-cell formation was only partly explained by a decreased rate of fertilization (63% vs 85%). However, subsequent development to expanded blastocysts was not different (75% vs 78%). An increased frequency of second polar body retention by fertilized frozen-thawed oocytes compared with controls (11·8% vs 1·3%) was shown to be largely responsible for the higher incidence of polyploidy (16·3% vs 3·7%). The frequency of polyspermic fertilization was not different in the two groups (3·9% vs 2·3%). Keywords: cryopreservation; in-vitro fertilization; mouse; oocyte; digyny; polyploidy