Growth enrichment studies utilizing in vitu cage cultures and a shipboard flowing seawater culture system were conducted with whole-plant populations of pelagic Sargassum-S. natans and S. fluitans (Phaeophyceaa)-in the western Sargasso Sea and at Looe Key Marine Sanctuary adjacent to the Straits of Florida. Growth rates of both species ranged from 0.03 to 0.04 doublings d −1 in control cultures receiving no enrichment and in cultures receiving either NO 3 − or NH 4 + enrichment; in contrast, growth rates ranged from 0.05 to 0.08 doublings d −1 in cultures receiving PO 4 3− enrichment. Midday photosynthetic rates of S. natans and S. fluitans were also two-fold higher with PO 4 3− enrichment, ca. 2.3 mgC g dryw−1 h −1 compared to 1.0 to 1.5 mgC g dry wt −1 h −1 in the NH 4 +, NO 3 −, and control treatments. These data suggest the pelagic Sargassum may grow faster than previously thought and that phosphorus, rather than nitroge, may be the primary nutrient limiting growth and productivity of these plants.