In Lake Mascardi (Patagonia), 2 acanthocephalan species, Acanthocephalus tumescens and Pseudocorynosoma sp., share an amphipod intermediate host but have different definitive hosts. Because both acanthocephalan species are potentially capable of manipulating amphipod behavior, one of the parasites may, therefore, have no opportunity to complete its life cycle; accordingly, negative interactions between them can be expected. The purpose of the present work was to examine the possibility of competition in the intermediate host through a comparison of A. tumescens and Pseudocorynosoma sp. cystacanth volume. Specimens of the amphipod Hyalella patagonica were collected monthly over almost 2 yr. Amphipods were measured (total length), necropsied, and cystacanths collected. Cystacanths were also measured, and their volume was calculated. Size of both acanthocepalan species was positively associated with amphipod total length. Competition, during 3 different infection periods, was assessed: high level of Pseudocorynosoma sp. infection (HP), high level of A. tumescens infection (HA), and high level of mixed infection (HM). In Pseudocorynosoma sp., intra-specific competition in HM was the only interaction found. In contrast, in A. tumescens, inter-specific competition in HP, intra-specific competion in HA, and intra- and inter-specific competition in HM were found. We suggest that Pseudocorynosoma sp. is a non-plastic species mostly found in single infections, while A. tumescens is a more variable species occurring more frequently in co-infections.