The carbon, phosphorus, and water cycles of a subtropical floodplain forest, and related ecosystem characteristics, were studied. Located at 750 m elevation in Puerto Rico (latitude 18°N) the forest had 27 tree species, 3059 stems/ha, a basal area of 42.4 m2/ha, maximum height of 17 m, and a leaf area index of 3.3. Palm (Prestoea montana) dominated the forest, and, with two other species, accounted for 68% of the dominance. Throughfall, stemflow, interception, runoff, transpiration, and evapotranspiration accounted for 81.6, 9.8, 8.6, 77.7, 13.7, and 22.3% of annual rainfall (3725 mm) respectively. The stand carbon storage was 28.77 kg/m2, distributed as follows: aboveground 35.4%, vegetation 44.2%, soil to 1 m depth 55%, palms 10.9%. Vegetation biomass was partitioned as follows: leaves 9.8% (75% are palm leaves), wood 68%, and roots 21.8%. Net aboveground primary carbon productivity (NPP) was 876 g°m—2°yr—1. Average litterfall was 2.4 g°m—2°d—1 (palm leaves 32%, other leaves 39%, and wood 10%). Half—lives of decaying material were 188, 306, 462, and 576 d, respectively, for palm leaves still attached to the parent tree, dicotyledonous leaves, palm leaves on the ground, and palm trunks. Total organic carbon concentrations in stream water increased with increasing stream discharge (from 2 g/m3 to 30 g/m3). Watershed export of carbon was 50 g°m—2°yr—1 (including 12 g°m—2°yr—1 in the form of leaf litter). Mean P concentration in palm leaves (1.18 mg/g) was twice that in dicotyledonous leaves (0.64 mg/g). Compared to a rainfall phosphorus input to the watershed of 63 mg°m—2°yr—1, leaching from the canopy was high (167 mg°m—2°yr—1), as was the loss of P from the watershed (611 mg°m—2°yr—1). Phosphorus—conserving mechanisms included a high rate of retranslocation in palms (504 mg°m—2°yr—1). In spite of these mechanisms, there was a net P loss from the watershed that ranged from 136 to 544 mg°m—2°yr—1. Periodic flooding, poor soil aeration, intensive year—round rainfall, and low atmospheric saturation vapor pressure deficits are believed to be the main driving forces of the floodplain forest, which exhibits many characteristics typical of lowland rain forests and floodplain wetlands. Rates of NPP, litterfall, and biomass turnover (residence time of 14—17 yr) are faster than expected for the climatic conditions, whereas rates of wood production and storage of organic matter in the vegetation and soil profile are lower than expected for the climate.