Summary. Unfertilized mouse oocytes were frozen and stored at –196°C. Immediately after thawing 331 out of 463 MF1 oocytes (71·5%) and 271 out of 410 F1 (C57BL × CBA) hybrid oocytes (66·1%) were morphologically normal. No significant difference was found between the survival of oocytes frozen and thawed with (70%) or without (66%) the surrounding cumulus cells. Fertilization in vitro of frozen–thawed oocytes was significantly lower than that of freshly collected control oocytes. The overall fertilization rate in vitro for MF1 oocytes was lower than with F1 (C57BL x CBA) hybrid oocytes. The presence or absence of cumulus cells during fertilization in vitro did not affect the fertilization rate. Live 14-day fetuses were obtained after transfer of frozen–thawed unfertilized oocytes directly to the oviducts of females mated with fertile males. However, much higher rates of survival (up to 45%) to 14-day fetuses and liveborn were found after the fertilization of frozen–thawed oocytes in vitro and subsequent transfer at the 2-cell or blastocyst stage to pseudopregnant recipients.