Yes, but there is a permit system. Information Bulletin #46 is for caches, but the guidelines for letterboxes are interpreted in the same manner. These rules became effective January 1, 2005. Each state park has an interpreter with discretion in administering the policy. The number of permitted caches and letterboxes is determined by density, and depends upon the available land area within each property. Placing letterboxes in dedicated nature preserves or other ecologically sensitive areas is not permitted.

Brad Bumgardner, park interpreter for Pokagon State Park, contacted speedsquare August 24, 2006 about an un-permitted letterbox located in an acceptable location within the park. Several informative and friendly emails were exchanged. Pokagon permitted a gathering of geocachers. A letterboxing event with letterboxes permitted for a short period of time may be permitted with prior approval.