Seedlings of 2 varieties of honey mesquite (Prosopis glandulosa var. glandulosa and P. glandulosa var. torreyana) were exposed to 2 concentrations of atmospheric carbon dioxide (CO₂) (368 and 704 μmol · mol⁻¹) in environmentally controlled glasshouses under near-optimal temperature and soil water conditions to determine if CO₂ enrichment alters above- and belowground growth responses. CO₂ enrichment substantially enhanced both above- and belowground growth variables of both varieties for all harvest dates (8, 16, and 24 days postemergence). This growth enhancement was greater for aboveground variables (21%–35%) at the first harvest, greater for belowground variables (36%–40%) at the second harvest, and similar for both above- (13%–68%) and belowground (10%–40%) variables at the last harvest. Differences in temporal growth enhancement associated with CO₂ enrichment suggest changing carbon allocation priorities, with initial carbon investment allocated primarily aboveground to develop photosynthetic machinery, and later carbon allocations predominately directed toward increased investment in roots. The absence of significant CO₂ × variety interactions at any harvest date provides evidence that CO₂ enrichment did not exaggerate growth responses between the 2 varieties. These results suggest that varietal differences in rooting and other characteristics did not modify the size advantage of the glandulosa over the torreyana variety, as the absolute differences in sizes did not change as a function of CO₂ treatment. Although CO₂ enrichment did not exaggerate growth differences between varieties in this species, it is evident that honey mesquite seedlings possess the capacity to respond markedly to CO₂ enrichment. The greater root depth of honey mesquite seedlings exposed to CO₂ enrichment confers a competitive advantage to mesquite seedlings over grass seedlings, assuming that C₃ and C₄ grass seedlings will not respond as vigorously to CO₂ enrichment. As such, this species should continue to aggressively encroach into grasslands in future CO₂-enriched environments.