The Pacific fisher (Martes pennanti) is an arboreal mesocarnivore perhaps best known for its elusive behavior and strong association with mature forest habitats.  As a result of the combined impacts of historical fur-trapping and habitat loss, the fisher no longer occurs throughout much of its former range in California and other western states.   Of particular conservation concern is a small population in the southern Sierra Nevada which has become geographically isolated from other populations.  Although a number of studies have characterized habitat associated with rest structures and home ranges, little information is available regarding the habitat requirements of reproductive females in this region.  As part of a larger project on fisher ecology, we are using radio-telemetry to monitor adult females during the denning season to identify the types of structures and habitat they require to successfully raise their kits.  We plan to investigate habitat characteristics associated with den structures at several spatial scales in order to identify how the sites that females are selecting differ from the surrounding forest.  Gaining a better understanding of the habitat needs associated with fisher reproduction will provide wildlife biologists and land managers with more information to assist in deciding how to best conserve habitat for this rare species.