Host specificity of Mediorhynchus centurorum for Melanerpes carolinus is thought to result from differences in nesting sites, nest sanitation, foraging behavior, and food items among woodpeckers (Picidae). Location of nests in dead limbs with intact bark and gleaning techniques in foraging make woodroaches, intermediate hosts of M. centurorum, a principal dietary item of Red-bellied Woodpeckers. The original description of Mediorhynchus centurorum reported a high degree of specificity for the vertebrate host. A survey of 68 avian species in 30 families revealed M. centurorum only in Red-bellied Woodpeckers, Melanerpes carolinus (= Centurus carolinus), although seven other species of woodpeckers were examined, including four from a small area where 10 of 16 M. carolinus found were infected (Nickol, 1969). Later, in demonstrating the life cycle of M. centurorum, cystacanths from woodroaches, the intermediate host, fed to four species of woodpecker resulted in patent infection of every bird (Nickol, 1977). Studies reported in this paper were undertaken to determine ecological factors that may have restricted the acanthocephalan species to Red-bellied Woodpeckers in the wild. MATERIALS AND METHODS Woodroaches (Parcoblatta pensylvanica) were collected from the vicinity of a small swamp in Iberville Parish, Louisiana, where Mediorhynchus centurorum occurred in more than 60% of the Red-bellied Woodpeckers. The prevalence of infection of woodroaches was determined for each site from which they were collected. Activities of woodpeckers at nests in Mississippi and Kansas were studied by procedures described by Jackson (1970a, 1976, 1977). Food items brought to nestlings were identified either from the ground with a 20X spotting scope or by observation from blinds through openings into nests. Frequency data were analyzed using the chisquare test, and a Student's t-test was used to compare sets of measurements. RESULTS AND DISCUSSION The distribution of larvae of Mediorhynchus centurorum in woodroaches was studied in a small, densely wooded swamp and surrounding pasture with scattered willow trees. Roaches were abundant under bark of dead limbs and fallen branches in both the swamp and pastu e. None of 139 from the swamp was infected with M. centurorum, whereas 8 of 89 from the pasture harbored larvae of that acanthocephalan. Because of all nests of Redbellied Woodpeckers were in the pasture and those of other woodpecker species of the area (Colaptes auratus, Dryocopus pileatus, and Picoides pubescens) were in the swamp, it seemed likely that parasite transmission to Redbellied Woodpeckers and woodroaches occurred at the nest sites. Red-bellied Woodpeckers do not practice strict nest sanitation; feces mixed with wood chips and bits of lost food in the dampness of the nest cavity make it an ideal chamber for survival of parasite eggs and woodroaches. Red-bellied Woodpeckers foraged around their nests and with other woodpecker species in the swamp. Apparently eggs are not sufficiently concentrated away from nests for woodroaches to become infected in appreciable numbers. Woodroach fragments were found in alimentary tracts of some individuals of all species examined from the area, but spatial separation of nesting sites and lack of nest sanitation are thought to be factors operative in the specificity exhibited by M. centurorum. In an attempt to identify ecological factors more precisely, nesting and foraging activities of Red-bellied Woodpeckers were compared with those of Red-headed Woodpeckers, Melanerpes erythrocephalus, because the two are closely related, similar in size, and nest along edges of forests rather than within them, and locate nests in limbs of similar diameters and heights (Jackson, 1976). Moreover, Nickol (1977) showed that Red-headed Woodpeckers are susceptible to laboratory infection with Received for publication 3 January 1978. * Department of Zoology, Mississippi State University, Mississippi State, Mississippi 39762. ti School of Life Sciences, University of NebraskaLincoln, Lincoln, Nebraska 68588.