Banner-tailed kangaroo rats ( Dipodomys spectabilis) are known to store large quantities of seeds in underground caches. Over 20 species of moulds have been found in the caches, and experiments have shown that the kangaroo rats prefer to ingest slightly mouldy seeds over non-mouldy and very mouldy seeds. Laboratory experiments were designed to test the hypothesis that these rodents manage seeds to promote an intermediate level of seed mouldiness. The experiments revealed that rodents tended to move sterile seeds to the highest humidities available. The same individuals moved seeds with the preferred level of mouldiness to humidities below the threshold for further mould growth. We interpret these activities as adaptive attempts to increase mould growth on sterile seeds and inhibit further growth on sufficiently mouldy seeds.