Reasonable Mitigation Act of 1999

Reasonable Mitigation Act of 1999 - Amends the Endangered Species Act to provide that mitigation of effects on endangered or threatened species or critical habitat shall not be required for any covered agency action and the Secretary of the Interior or Commerce, as appropriate, may not require compliance with specified measures to minimize impacts, if the aggregate cost of the mitigation, implementing such measures, and complying with conditions for implementing such measures for the action will exceed ten percent of the total project costs of activities comprising or authorized by the action. Prohibits the Endangered Species Committee from establishing any mitigation and enhancement measures for such actions if the cost of implementing such measures will exceed ten percent of total project costs. Defines "covered agency action" as any construction project carried out or funded by a Federal agency and the issuance by a Federal agency of a permit that would authorize a State or local construction project.

Prohibits the Secretary from requiring a permittee for a public project to take any measures to minimize or mitigate impacts of an incidental taking of an endangered species under the permit if the costs of such measures will exceed ten percent of total project costs.