A method is described by which the rate of degradation of different kinds of cellulose fibers was determined in aquatic environments. The fiber samples were enclosed in small bags of nylon fabric with a fine mesh and weight losses of bags placed in water or sediments were measured over periods of several months. Newly processed pulp fibers were rapidly degraded in aerobic, nutrient-rich water whereas previously dried pulp and fibers derived from cotton were degraded more slowly. Lignin-containing mechanical pulp was hardly degraded at all. Cellulose decomposition was slow in unpolluted sea water and in the heavily polluted, anaerobic water outside a paper mill but relatively rapid both in the anaerobic water and the sediment of a nutrient-rich lake. The applications and limitations of the bag method in studies of decomposition processes in aquatic ecosystems are discussed.