This is a study of the vascular plants occurring in Oxford County, Maine. Plants with vascular or conductive tissue (kingdom Plantae, division Tracheophyta) include the ferns and their "allies" (horsetails and lycopods) and the seed-bearing plants. Our aim has been to determine which plants occur in the county and, based on herbarium collections and observations, their habitat preferences, frequency of occurrence, and geographic distribution. We have also endeavored to explain the nature of the flora first in terms of the various factors which influence its composition and second by grouping the plants into the most important natural associations