PROCEEDINGS OF THE BIOLOGICAL SOCIETY OF WASHINGTON 110(4):569-580. 1997. Aglaodiaptomus atomicus, a new species (Crustacea: Copepoda: Calanoida: Diaptomidae) from freshwater wetland ponds in South Carolina, U.S.A., and a redescription of A. saskatchewanensis (Wilson 1958) Adrienne E. DeBiase and Barbara E. Taylor The University of Georgia, Savannah River Ecology Laboratory, RO. Drawer E, Aiken, South Carolina 29802, U.SA. Abstract. — A new calanoid copepod, Aglaodiaptomus atomicus, is de-scribed from freshwater wetland ponds in Aiken County, South Carolina, U.S.A. It is common in Aiken and Barnwell Counties. It has been collected from shallow, acidic wetland ponds, most of which dry periodically. Agla-odiaptomus atomicus resembles A. saskatchewanensis, which is redescribed from Saskatchewan, Canada, and Louisiana, U. S. A. The male of A. atom-icus differs from all of its congeners by possessing a distinctively large distolateral process on the right leg 5, exopod 1. The female differs from its closest congener, A. saskatchewanensis, in possessing nearly symmetrical thoracic wings. In A. saskatchewanensis , the wings are distinctly asymmet-rical. A survey of freshwater wetland ponds on the upper coastal plain of western South Carolina revealed an apparently unde-scribed calanoid copepod species. This spe-cies, which was incorrectly reported as Diaptomus conipedatus Marsh, 1907 by Mahoney et al. (1990), is one of the most common calanoid copepod species occur-ring in Carolina bays and other isolated, shallow, wetland ponds in Aiken and Barn-well Counties, South Carolina. It most closely resembles Aglaodiaptomus sas-katchewanensis Wilson, 1958. However, detailed comparisons with specimens of A. saskatchewanensis proved it to be a sepa-rate, undescribed species. In her original description of Agla-odiaptomus saskatchewanensis, Wilson (1958) provided an illustration of only the male fifth leg and a partial description of the species. Few additional details were added to her 1959 key (Wilson 1959). A more detailed description of A. saskatch-ewanensis is presented here, as well as a description of the new species, A. atomi-cus. Diaptomid copepods vary in the follow-ing morphological characteristics: body length, features of the thoracic somites (especially the 6 th somite), length, number of and armature of the antennules in both sexes, number of somites of the female urosome, and segmentation, shape and or-namentation of the fifth leg of both sexes (Wilson 1959). The mouth parts and legs 1-4 of the species presented in this paper correspond to those of Leptodiaptomus si-ciloides (Comita & Tommerdahl 1960). Descriptions and measurements of whole copepods were made in glycerine. Dissect-ed specimens were mounted either in lac-tophenol or in CMC-9. Most were stained with Chlorazol Black E before being mounted.