The Secretary of the Interior, acting through the Fish and Wildlife Service, is directed by the Endangered Species Act to designate critical habitat concurrently with the listing of a species as endangered or threatened. However, the ESA allows FWS to delay critical habitat designation upon a finding that designation is not prudent or that it is not determinable. FWS has liberally exercised these exceptions to avoid designating critical habitat for the majority of listed species. In response, citizen groups regularly file suit to compel designation. Difficulties arise when the failure to designate occurred more than six years before the filed action. Some federal courts hold the general civil statute of limitations, 28 U.S.C. § 2401(a), bars actions to compel designation. Others have relied on principles of equitable tolling to allow actions to go forward. This Note argues that courts should toll the statute of limitations in actions to compel designation where FWS made a “not determinable” finding because it constitutes a failure to act despite a non-discretionary, mandatory duty, but that “not prudent” findings constitute final agency action and should start the clock running for statute of limitations purposes.