The correspondence files are arranged alphabetically; most of them have the name of the person writing, but some files have subject titles. Some letters have been arranged in chronological files, but all have similar subjects. Mulford and Racine Foster corresponded with individuals and organizations around the world about all aspects of their business including identifying and selling bromeliads. They corresponded with experts at such institutions as the Smithsonian and Kew Gardens as well as local nurseries. The series also includes personal correspondence, Christmas cards and letters to publishers. Further correspondence with publishers may be found in its own subseries. The folder entitled "Lectures" contains arrangements for Mulford Foster to lecture not the lectures themselves, which may be found in the Lectures subseries. Correspondence between Racine and Mulford includes various other writings by them both. Some letters include photographs; Tom Lineham returned some photographs to Racine Foster within his letter and Kay Schools illustrated her letters with drawings of bromeliads. Correspondence sometimes continued for decades, but often a folder contains only a single letter. Occasionally, both sides of the correspondence is available, which shows that Mulford and Racine frequently responded to each other's letters. Family correspondents include Racine's sisters Jeanne Denny and Florence Hall and Mulford's brothers Hubert and Preston and his sisters May Hurff and Rosena Whitlock. Notable correspondents include David Fairchild, A. J. Hanna, F. C. Hoehne, Harry Leu, Victoria Padilla and Lyman B. Smith.
