This paper describes the denning behaviour of 24 feral ferrets Mustela furo on farmland, East Otago, South Island, New Zealand. One hundred and ninety seven dens were located and radio-collared ferrets were found to share dens simultaneously with other ferrets on 7.4% of 706 radio-tracking events and used dens that had been previously used by other ferrets (sequential den sharing) on 44.3% of occasions. In this particular study, ferrets were exhibiting a higher degree of sociality than has been recorded in other solitary mustelid species and therefore may not be adhering to the model of intrasexual territoriality thought to apply to other mustelid species. Den sharing may be a mechanism by which bovine tuberculosis Mycobacterium bovis is transmitted within ferret populations. Over 80% of the sequential den sharing occurred within 14 days, well within the survival span of M. bovis bacilli. Denning of infected ferrets in haybarns may pose a risk of transmission of M. bovis to livestock from hay.