SUMMARY From 20 to 50% of sexually mature female mice, grouped together in large cages but isolated from males since weaning, were found to be pseudopregnant as indicated by abeyance of oestrus, vaginal mucification, weight changes and mammary development. Such 'extra-coital' pseudopregnancies occurred independently of experimental handling in grouped females but were also observed, although much less frequently, in isolated mice subjected to daily examination. The duration of pseudopregnancy was found to be longer and much more variable than has previously been suggested.