Adults of the caryophyllaeid cestodes Biacetabulum infrequens Hunter and B. macrocephalum McCrae occur in catostomid fishes and procercoids develop in annelids of the genus Tubifex, but not in those of Limnodrilus. Procercoids are characterized by a cercomer, primordia of gonads, and a well-developed scolex. B. macrocephalum procercoids are infective to fish at 62 days. The genus Archigetes is distinguished from Biacetabulum on the basis of differences in morphology, definitive hosts, intermediate hosts, and larval development. Life cycles for tapeworms of the caryophyllaeid genus Biacetabulum Hunter have not been previously reported. Also, some workers consider certain species of the related genus Archigetes Leuckart as members of Biacetabulum. This study, concerning B. infrequens Hunter and B. macrocephalum McCrae, was conducted in order to elucidate the biology of Biacetabulum and to determine its relationship to Archigetes. A supplementary study regarding temperature stress on procercoid development in B. macrocephalum was reported by Buchwald and Ulmer (1964). MATERIALS AND METHODS Fishes were collected from the Iowa River (Franklin and Hardin counties) in Iowa. Specimens examined included 24 Carpiodes carpio (Raf.), 6 C. cyprinus (LeSueur), 11 C. forbesi Hubbs, 5 C. velifer (Raf.), 339 Catostomus commersoni (Lacepede), 61 Hypentelium nigricans (LeSueur), 33 Ictiobus cyprinellus (Val.), 6 Moxostoma anisurum (Raf.), 96 M. erythrurum (Raf.), 90 M. macrolepidotum (LeSueur), 317 Cyprinus carpio L., 7 Hybopsis biguttata (Kirtland), 12 Notropis cornutus (Mitchill), 8 N. dorsalis (Agassiz), 7 N. spilopterus (Cope), 9 N. stramineus (Cope), 10 N. topeka Gilbert, 12 Pimephales notatus (Raf.), 10 P. promelas Raf., and 10 Semotilus atromaculatus (Mitchill). Received for publication 24 August 1964. * 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. All experiments and observations were conducted at room temperature (18 to 22 C) unless otherwise specified. Cestode eggs, obtained by keeping gravid worms in distilled water or by dissection, were allowed to develop in stream water. Oligochaetes were exposed to embryonated eggs in a mud medium. After 1 to 3 days exposure, annelids were removed and maintained in a separate container. All annelids were cultured in mud under aeration and were fed a mixture of oatmeal, cornmeal, and commercial fish food. Annelids of the genus Tubifex used in experiments were laboratory reared while those of Limnodrilus were collected in nature and held in the laboratory at least a month prior to use. Cestodes were fixed without pressure in AFA and annelids were preserved by pipetting them into hot 70% ethanol. Carmine stains were used for whole mounts and hematoxylins were used for sections. Egg and oncosphere measurements were made in water on 20 eggs taken from several cestodes. Sample size (N) is given for other measurements.