In either single-species or concurrent infections, both helminths become established near the middle of the intestine, later moving anteriorly. In simultaneous concurrent infections, or patent infections challenged by M. dubius, the H. diminuta later retreat to the middle third of the intestine. In the presence of mature M. dubius, developing H. diminuta do not migrate anteriorly. The M. dubius are not markedly influenced in their position or rate of migration by concurrent infection. H. diminuta reach a maximum size in 3 weeks, followed by irregular fluctuations in length or wet weight. In concurrent infections, growth is retarded to a degree correlative with the weight of M. dubius present in the same or anterior regions of the intestine. M. dubius is characterized by an arithmetical growth phase persisting throughout the experimental period (10 weeks). In concurrent infections, gain in weight is reduced to a degree commensurate with the weight of the H. diminuta present Gain in length is retarded only by mature or almost mature H. diminuta in the same or anterior regions of the intestine. Both species are initially attenuate, later becoming more robust (higher weightlength ratios). The weight: length ratio of the H. diminuta reaches a constant level at maturity; in simultaneous concurrent infections this ratio reaches a maximum at maturity (3 weeks), then decreases. In the acanthocephalans, the increase continues throughout the period studied. The rate of increase for either species is retarded in the presence of the other. The growth rate reductions and changes in body shape are similar to those observed in cases of intraspecific crowding and to those observed in tapeworms from rats fed suboptimal carbohydrate diets. The similarities suggest that competition for carbohydrate may be the mechanism underlying the effects of concurrent infections.