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Body size influences virtually all aspects of the autecology of material flow by organisms. Thus, size classes may be considered as alternatives to grouping by trophic level, function or taxonomy to describe the structure and function of communities. The general intent of this thesis is to assess the role of body size in the dynamics of radiophosphorus in the freshwater plankton. To calculate a priori estimates of the fluxes of phosphorus through size classes of plankton, the rates of assimilation and excretion of laboratory cultures of algae and cladocerans were measured, and regressed on body mass (W). In both groups, assimilation increased as W('0.75). The turnover of body phosphorus was described as a two-compartment system. The turnover rates of the small (2-5% of body P) fast turnover pool, the large (95-98%) slow turnover pool, and that of the body phosphorus fell proportionately to W('- 1/4) in algae and W('- 1/2) in zooplankton. Field tests of allometric models of radiophosphorus flow were conducted by following the redistribution of radioactivity among 11 size fractions of enclosed epilimnetic assemblages over 5 to 6 days. Concentration of the tracer in the biomass declined with size soon after addition and gradually shifted to a uniform distribution within 3 days. A model that makes no a priori assumptions as to the trophic structure of the system, i.e. all size classes are at the same trophic level, predicted these dynamics better than one that assumes trophic position to increase with size. Further analyses indicated that explicit treatment of the physiological allometry and size distribution of broad functional groups of plankton are needed to improve the descriptive power of the model further.