The ability of the Rottnest Island population of the quokka (Setonix brachyurus) to replace lost pouch young from stored blastocysts has been examined. It was found that, under experimental conditions, young were replaced throughout the breeding season. However, the age distribution of the young in the wild population provided evidence that there was no considerable replacement from stored blastocysts in the field. The maximum replacement under experimental conditions was 60 per cent at the beginning of the breeding season in February. The percentage of animals producing further young declined gradually after this until in August, when seasonal anoestrus commences, no young removed from the pouch were replaced from stored blastocysts. Factors which may affect the ability of the animals to produce young from stored blastocysts, and the decline in success throughout the breeding season are discussed.