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The importance of waterhyacinth plant material (living and detrital) as a sink for nitrogen (N) and phosphorus (P) was evaluated in a waste water treatment system in south Florida. The treatment system consisted of four ponds (0.1 ha each) from which plants were harvested, and one pond (0.2 ha) from which plants were not harvested. Nitrogen and phosphorus were removed in the plant standing crop at higher rates in the harvested ponds (362 and 115 mg m−2 d−1, respectively) than in the non‐harvested pond (55 and 15 mg m−2 d−1, respectively). However, immobilization of N and P as plant detritus in the sediment was less in the harvested ponds (3 and 1% of standing crop assimilation, respectively) than in the non‐harvested pond (33 and 13% of standing crop assimilation). With the exception of N in the nonharvested pond, estimates of nutrient removal by the waterhyacinth standing crop and detritus from 4 May–4 Aug. 1981 were similar (within 35 mg N or P m−2 d−1) to the observed nutrient removal from the waste water. Denitrification accounted for 92% of the N removed from the non‐harvested pond. On an areal basis, approximately 2.4 times more N was removed by denitrification in the non‐harvested pond than was assimilated by waterhyacinth growth in the harvested ponds (875 vs. 362 mg N m−2 d−1).Mean total N removal from the complete system resulted in an 87% reduction, from 9.96 to 1.25 mg/L. Total P was reduced only 10%, from 4.68 to 4.23 mg/L.