The aquatic pteridophyte Azolla, a small-leaf floating plant, which lives in symbiosis with a nitrogen fixing cyanobacteria, Anabaena azollae, was widespread throughout water channels and hydrographic basins of Portugal. Azolla is also the aim of a study for its utilization as a biofilter for wastewater purification, namely for phosphorus removal (+/- 36%). The goal of this work is to compare the growth characteristics and biomass composition of this water fern in natural ecosystems with those obtained in some wastewaters. Plant growth rate (0.107 +/- 0.037 d(-1)) and productivity (5.8 g dw m(-2) d(-1)) suggest that Azolla can grow well in partially treated domestic wastewater, but not in diluted pig wastes. This fact, associated to its biomass composition, namely in phosphorus content (1.38 +/- 0.20%), increase the possibility of this plant being used to improve wastewater discharge quality. It may also be possible to use the biomass as a biofertiliser or as a feed supplement for aquatic and terrestrial animals due to its protein, crude fiber and mineral content.