The pleopods of the river crabs (Potamonidae) are indispensable to the taxonomy of the group. Modern workers have been careful to illustrate them, but detailed descriptions are usually lacking. The apparent complexities of the gonopods (a more convenient term than male pleopod) make them difficult to describe and compare. Partly as a result of failure to compare gonopod morphology, no division has ever been made of the unwieldy genus Pseudothelphusa (about 75 named species). Rathbun (1898) listed types of gonopods but did not relate them to each other, nor did she propose a formal subgeneric classification. The recognition of subgenera, and of the proper relationships between genera and subfamilies, must depend on an understanding of the gonopodal structure. To this end, some terms are proposed for the most constantly occurring features of river crab gonopods. These remarksapply only to the subfamily Pseudothelphusinae, i.e., the New World river crabs exclusive of the genus Trichodactylus. Bott (1955) proposed family rank for the Pseudothelphusinae, a proposal supported by the marked difference in structure between the gonopods of Bott's Pseudothelphusidae and those of the Potamonidae. Of the five genera of the Pseudothelphusinae, representatives of Pseudothelphusa, Potamocarcinus, and Epilobocera were examined in preparing this paper; I have not examined specimens of the monotypic genera Rathbunia and Typhlopseudothelphusa. A standard terminology of crawfish gonopods, largely devised by Hobbs (1940, 1942, 1945), has proved very useful and can be applied in part to river crab gonopods. In using Hobbs' rather noncommittal terms, no homologies are suggested between the gonopods of crawfishes and those of river crabs. In addition, the gonopods of crawfishes and brachyurans are not homologous with the petasma of Penaeidae, and the term petasma should be restricted to the copulatory endopods of the Penaeidae. All the remarks below refer to the first pleopods. Naming the gonopodal surfaces presents no particular problem. As the first pleopod lies against the abdomen, the side facing the observer has usually been called the outer, external, or abdominal surface, but will here be designated the cauctal surface since, if the abdomen were in an unflexed position, this surface would, in fact, face caudad. Similarly, the other sides will be referred to as the cephalic, lateral, and mesial surfaces. These terms are chosen instead of sternal and abdominal surfaces, as employed by Gordon (1937) and Tweedie (1940) for Grapsidae, and Garth (1958) for Oxyrhyncha. A gonopod which has been removed from the crab can be oriented by finding the margin (see below), which can always be considered caudal, at least in the proximal portion of the gonopod. Sometimes the margin faces somewhat mesiad, but for the purposes of the proposed terminology, the margin is always caudal. The coxopodite and basipodite are clearly evident in both the first and second gonopods. The distal segment, sometimes called a flagellum in brachyurans, is typically thick and blunt in the Pseudothelphusinae and cannot be homologized with either endopodite or exopodite (Cochran, 1935). Figure 1-A shows a cross section of the distal segment of the right first gonopod of Pseudothelphusa cobanensis Rathbun,