The survival rate and development of four-cell-stage mouse embryos frozen and thawed in S phase versus G2 phase was compared. Significantly more G2-phase than S-phase embryos survived freezing and thawing. In both groups, disruption of the zona pellucida, fusion of blastomeres, and dispersion of chromosomes were occasionally observed after thawing. Cryopreservation resulted in a longer delay in cleavage from the four- to the eight-cell stage of S (about 5 hr)- and G2-phase embryos (about 3 hr) compared to unfrozen controls. The number of frozen embryos which developed to the blastocyst stage was reduced compared to controls, and in the case of S-phase embryos, formation of the blastocyst cavity was also delayed. However, the average number of cells in the experimental and control embryos was similar. No increased incidence of chromosome abnormalities was seen. Our results suggest that freezing embryos in G2 is superior to freezing in S phase.