The primary source of nutrients for seagrass growth is considered to be the sediment porewater. Porewater nutrient concentrations were measured in 18 seagrass beds across Florida Bay, USA, a shallow, seagrassdominated subtropical embayment, during the summers of 1987 and 1988. Concentrations of nutrients in porewater varied widely, with median values of 0.34 pM for soluble reactive phosphorus (SRP) and 78.6 gM for NH +. SRP and NH + concentrations were positively correlated. Due to spatial heterogeneity, there were no apparent trends with sediment depth (down to 40 cm) in the porewater nutrient concentrations. The SRP concentration of the porewater was highest in areas supporting Halodule wrightii, intermediate in areas of Thalassia testudinum, and lowest in sediments without seagrasses. There was no similar relationship with NH +. Porewater SRP, but not NH +, was significantly correlated with total seagrass standing crop. Elemental content (both N and P) of green leaves of T. testudinum was a function of the concentration of the nutrients in the porewater. Standing crop of T. testudinum was correlated with phosphorus content, but not with N content, of the seagrass leaves. The results support the hypothesis that sediment porewaters are the most important source of nutrients for seagrass growth. In this subtropical carbonate environment, the availability Of phosphorus in the porewater limits the development, and controls the species composition, of seagrass beds.