The life cycle of this parasite has been studied in relation to its intermediate host. Infection of the isopod population with worm larvae is initiated in the summer. By the following March, as many as 60% of the isopod population is infected. Male and female isopods are equally susceptible. The larger a female isopod is, the more likely it is to be infected. In contrast, the incidence of infection in male isopods increases with increasing size up to about 10 mm, after which the level of infection stabilizes at approximately 40%. Both male and female isopods carry significantly more worm larvae in the early part of the parasite's life cycle than in later months. Isopods that were heavily infected in the early part of the life cycle disappear from the population. It is suggested that heavily infected isopods suffer a mortality which regulates the isopod population density. The presence of worm larvae apparently causes a color change in the isopod host. This change presumably increases fish predation on infected isopods, thereby increasing the probability that the parasite's life cycle will be completed. Aside from studies of development in the invertebrate host and scattered observations of seasonal variation and incidence of infection, little is known about the general biology and host-parasite relationships of the American acanthocephalan fauna. The present study was undertaken to determine the host-parasite relationships between the isopod, Asellus intermedius Forbes, and the acanthocephalan, Acanthocephalus dirus (Van Cleave, 1931). Acanthocephalus dirus was originally reported by Van Cleave (1931) from the freshwater drum, Aplodinotus grunniens, in the Yazoo River in Mississippi. The freshwater drum was also found to be infected with A. dirus in Reelfoot Lake, Tennessee (Bangham and Venard, 1942). A subsequent report by Van Cleave and Townshend (1936) indicated its presence in the bluegill, Lepomis macrochirus, and the channel catfish, Ictalurus punctatus, in the Ohio River and one of its tributaries in southern Illinois, and from fish in the Illinois River and its tributaries near Havana, Illinois. The area of this study was, in fact, a small tributary whose waters enter the Illinois River Received for publication 12 January 1973. * Based upon a portion of a Ph.D. thesis submitted to the Department of Zoology, University of Illinois. t Present address: Department of Biology, Virginia Commonwealth University, Richmond, Virginia 23220. just south of Havana. Adults of this acanthocephalan were recovered from the bluntnose minnow, Pimephales notatus, and a chub, Hybopsis sp. MATERIALS AND METHODS The population of isopods studied was that of Mud Creek (T22N, R2E, Sect. 31) in McLean County, Illinois. This creek is approximately 3 miles long. It flows through pasture and cornfields before its confluence with Kickapoo Creek, which eventually flows into the Illinois River. Sampling stations were set up at Mile 1 and Mile 2, numbered from the source of the stream. The population was surveyed qualitatively from 31 January 1967 to 29 August 1968 by scooping the rooted aquatic vegetation and vegetation overhanging the banks with a flour strainer (4.33 mesh per cm). This material was brought back to the laboratory and within 24 hr isopods were sorted out from the vegetation and debris and preserved in 70% ethyl alcohol. Isopods less than 4.0 mm long were considered to be juveniles. All isopods over 4.0 mm long were sexed and measured to the nearest 0.1 ml with a binocular microscope fitted with an ocular micrometer. The length of the isopod was the distance from the anterior margin of the cephalothorax to the posterior margin of the telson. Each isopod was then examined for the presence of acanthocephalan larvae in the body cavity. Four times during the study a special collection was made. The isopods were sexed and measured, the number of worm larvae were counted, and the isopod and its worm larvae were dried at 65 C for 48 hr and then weighed on an analytical balance. From these data, least squares regression lines were calculated for the relationship