Compass and sextant observations by Meriwether Lewis and William Clark are combined to provide the oldest determinations of the magnetic declination in the continental interior of the United States. Over the past 200 years, the magnetic declination near St. Louis has changed from an azimuth of 7.7° east to 0° today. The 1803–1806 declinations are essential to interpreting the travel legs made by Lewis and Clark on their historic journey, and could be used to test and improve existing magnetic models.