A floristic study of the vascular plants of Lost Valley, Newton County, Arkansas, in the Boston Mountains of the Interior Highlands, was initiated in April, 1974, and continued through May, 1975. Several mixed mesophytic Appalachian disjuncts and Ozarkian endemics are represented in five vegetation community types in the Oak-Hickory Forest Region. An annotated list of 658 taxa representing 372 genera of 104 families is included; these represent 261 Newton County records.