The black-footed ferret depends on prairie dogs for both shelter and food, living in their underground ‘towns’ and preying on them. But the prairie dogs, regarded as pests by the ranchers and farmers, have been subjected to intensive poisoning campaigns which successfully exterminated them over most of their range and heavily reduced their numbers in the rest. This has brought the ferrets to the verge of extinction. A conservation management programme is urgently needed but difficult to devise, because so little is known about these elusive, nocturnal and now very rare underground dwellers. The author believes that computer simulation models now offer a good study method, and suggests how this should be done.