The accompanying tabular data on the ragweed pollen hazards in the national parks were secured over a period of thirteen years, during which I made field studies and carried out routine atmospheric pollen tests for one or more seasons in each of the parks and areas herein listed, except as noted. Most of the atmospheric tests have been made by the standard technic recommended by the Aero-Allergen Council of the American Academy of Allergy.1The figures for "days of high incidence" refer to the average number of days per season when the ragweed pollen concentration reached or exceeded 25 pollen granules per cubic yard of air. "Maximum atmospheric concentration" refers to the highest number of ragweed pollen granules per cubic yard of air during any one twenty-four hour period. A "seasonal total" is the cumulative figure obtained by adding the number of ragweed pollen granules per cubic yard for