Current approaches to measure the activities of microbial extracellular enzymes in aquatic environments are hampered by slow throughput or by differences between the structure of simple substrate proxies and macromolecules. Here we show that measurements of fluorescence anisotropy can be used to determine the hydrolysis rate of two fluorescently labeled polysaccharides, laminarin and xylan, in environmental samples. A simple analysis shows that the anisotropy of these fluorescently labeled polysaccharides can be approximated using a modification of the Perrin equation.