Three species of cestodes, Hymenolepis diminuta, H. citelli, and H. microstoma, were shown to transport nucleosides by a mediated process. In H. diminuta the Ki values of various competitive inhibitors led to the conclusion that at least 2 different loci are involved in the transport of nucleosides. One of these loci has greater specificity for the purine nucleosides, the other for pyrimidine nucleosides. No significant difference was observed in their affinity for ribo- or deoxyribonucleosides, although thymidine was slightly less effective as an inhibitor of ribonucleosides than was uridine. Transport by the pyrimidine nucleoside locus was stimulated by thymine, but not by hypoxanthine, whereas transport by the purine nucleoside locus was stimulated by hypoxanthine but not by thymine. Preloading the worms with thymine gave less stimulation of transport than did the presence of eoxgenous modulator. Efflux of previously accumulated nucleoside was not blocked by the presence of exogenous modulator. The presence of exgenous thymine enhanced the incorporation of uridine into the nonextractable pool. Thymine also stimulated uracil transport in H. citelli, but not in H. microstoma, and had no effect on uridine transport in either of these species. The results of transport and modulator studies on H. diminuta grown in hamsters were not different from the results with worms grown in rats indicating that the regulatory effects observed were an inherent function of the parasite and not the host.