1. A simple and inexpensive assay was developed to determine the amount of biologically available phosphorus (BAP) present in freshwater.2. BAP was measured by using Aeromonas hydrophila in a proteolytic assay system in which the release of the chromogen from an insoluble azure dye derivative of hide powder (HPA) was determined spectrophotometrically. The time necessary to hydrolyse 50% of the HPA was found to be directly dependent on the amount of BAP present. BAP within the range 2-250 ^g 1~' phosphate-phosphorus was detected.3. Our data suggest that the application of this assay for the measurement of BAP in freshwater offers the potential of monitoring the biological response of a freshwater ecosystem to phosphorus.