This is the third revision of Keys to the Vascular Plants of the Black Gap Wildlife Management Area, Brewster County, Texas (Mahler 1969 – Ditto; Mahler 1970 – Xerox). The main objective of this study is to provide students of the flora of this area with a means by which the plants may be studied in a more efficient manner than has previously been possible. Over 400 species are included in this treatment. This should be regarded as a continuing study and is not a final, comprehensive treatment.