Elodea cut into two initial particle sizes (2.0 cm sections and 3.0 millimeter sq fragments) underwent microbial decomposition for periods of 0, 15, 33, and 45 days, and then was fed to four size classes of crayfish (2.0, 3.5, 5.0, and 9.0 cm). Assimilation efficiency was determined by Conover's method. All four size classes of crayfish assimilated 15-day detritus more efficiently than 0-day detritus. Two-cm crayfish assimilated 33-day detritus more efficiently than 15- or 0-day detritus. In all other cases, assimilation efficiency was reduced. Radiotracer techniques utilizing 14C were used to determine the percentage of carbon assimilated in the form of plan fragments compared with the percentage assimilated in the form of microorganisms. Results indicate an inverse relationship between crayfish size and the percentage of carbon assimilated in the form of microorganisms.