The life cycle of the caryophyllaeid cestode Archigetes iowensis was studied in naturally and experimentally infected hosts. Adults are seasonal parasites of the fish, Cyprinus carpio, while larvae occur in the seminal vesicles of the oligochaete, Limnodrilus hoffmeisteri. Procercoids are characterized by a well-formed scolex, fully developed reproductive organs, and by a cercomer. Larvae occasionally produce eggs within the annelid host. Feedings of infected annelids to fish did not provide conclusive evidence regarding the manner in which fish acquire their infections. The periodicity of adults in fish seems to result from changes in the resistance of carp to these helminths. Life cycles for members of the caryophyllaeid genera Archigetes Leuckart and Biacetabulum Hunter were studied in an attempt to resolve taxonomic problems. Archigetes iowensis Calentine, the subject of this report, normally produces eggs within a fish host, but procercoids occasionally exhibit progenetic development within the annelid intermediate host. Pertinent literature was cited earlier (Calentine, 1962); more recent accounts of Archigetes include Brinkhurst et al. (1962), Kulakowskaja (1962a, b), and Nybelin (1963). Studies on Biacetabulum and taxonomic considerations will be reported later. MATERIALS AND METHODS Natural history studies were conducted on the Iowa River (Franklin and Hardin counties, Iowa, USA). A total of 975 fishes representing 20 species was examined for the presence of intestinal parasites. Members of the cyprinid genera Cyprinus, Hybopsis, Notropis, Pimephales, and Semotilus and those of the catostomid genera Carpiodes, Catostomus, Hypentelium, Ictiobus, and Moxostoma were included. Carp (Cyprinus carpio L.), the Received for publication 10 October 1963. * Part of a doctoral thesis, Iowa State University. This study was supported in part by National Science Foundation Grants (G-9022 and B-6844) under the direction of M. J. Ulmer, and in part by a Public Health Service predoctoral fellowship (GPM-18,590) from the National Institutes of Health. t Present address: Department of Biology, Wisconsin State College, River Falls, Wisconsin. only fish host found for A. iowensis, were examined 9 months of the year (March through November) during the interval 1961 to 1963. Cestodes were fixed (without pressure) in AFA and oligochaetes were preserved in Bouin's fluid. Specimens were processed in the usual manner using carmine stains for whole mounts and hematoxylin for most sections. Egg development was followed in stream water at room temperature. Feeding experiments were conducted by mixing embryonated eggs with a small quantity of mud, to which 72 to 99 tubificids were added. After 1 to 3 days exposure, annelids were removed and maintained in a separate container. All oligochaetes were held in mud under aeration at room temperature and were fed a mixture of oatmeal, cornmeal, and commercial fish food. Tubificids used in experiments were either laboratory-reared specimens or worms collected in nature and maintained in the laboratory 3 to 6 months prior to use. Fish used in experiments were taken from waters where Archigetes was apparently absent. Small fish were allowed to ingest infected annelids while the larger fish were force fed.