Rates of extracellular amino acid oxidase activity in natural phytoplankton, cyanobacterial, and bacterial assemblages were measured using a fluorescent analog of the amino acid lysine. Activity was measured in a variety of ecosystems with different levels of nutrient enrichment and diverse community composition. Sites included a station in Shinnecock Bay, Long Island Sound, New York (USA); the Chesapeake Bay, Maryland (USA); the NW Atlantic Ocean near the Bahamas and the Caribbean Sea; Brazilian coastal waters; and 2 estuarine mesocosms. Highest rates of amino acid oxidase activity (25 to 30 nM h-') were found in the summer mesocosm experiments when NH,+ concentrations were near the limit of detection, and biomass levels were indicative of an algal bloom. Lower rates of amino acid oxidase activity were found during a bloom of Aureococcus anophagefferens and in oligotrophic oceanic waters. High rates of amino acid oxidase activity (up to 20 nM h-') were also found in oceanic samples enriched with colonies of the diazotrophic cyanobacteria Tnchodesmiurn. No activity was observed in samples from oligotrophic environments that were prefractionated through 1.0 pm filters; however, when amended with glucose or an amino acid mixture, oxidation rates of up to 8 nM h-' were observed. No activity was found during a diatom-dominated, autumnal bloom in Chesapeake Bay. Overall, amino acid oxidation represented a higher percentage of NH,' uptake in the oligotrophic waters (up to 10%) than in the coastal waters studied. In oligotrophic waters, where ambient inorganic nitrogen concentrations are low and consequently uptake rates are low, this pathway appears to represent a potentially important source of nitrogen for phytoplankton and the diazotrophic cyanobacteria Tnchodesmium.