On the Jornada Experimental Range in southern New Mexico, 2 belt transects, 30.5 cm in width and totaling 2,l88 m length, were established in 1935 on 2 areas where honey mesquite (Prosopis glandulosa Torr.) was spreading into black grama [Bouteloloua eriopoda (Torr.) Torr.] grassland. Maps were made of the transects which portrayed the vegetation occurring in each of the 7,180 contiguous, 0.09-m²  plots along the transect. The vegetation on the transects in 1980 was compared to that portrayed by the transect maps made in 1935. One transect bad been read in 1950 and 1955. During the 45-year period mesquite attained complete dominance and many new mesquite dunes formed. Black grama had a relatively high frequency in 1935 but had completely disappeared by 1980, both on an area grazed by livestock and on an area protected from grazing. Mesa dropseed [Sporobolus flexuosus (Thurb.) Rydb.], fluff grass [Erioneuron pulchelbum (H.B.K.) Tateoka] and broom snakeweed [Xanthocephalum sarothrae (Pursh) Shinners] increased in abundance, even during the drought period between 1950 and 1955. Only 25% of the perennial forb species encountered in 1935-55 were found in 1980.