The yellow mud turtle (Kinosternon flavescens) is a xerothermic relict partially distributed among several disjunct populations on sand prairies along the former Prairie Peninsula in Illinois, Iowa, and Missouri. In Illinois, where the species is listed as endangered, the largest known population occurs at the northeastern extent of the species' range in Henry County. A major portion of the site is in private ownership, and the Illinois Department of Natural Resources seeks to acquire additional mud turtle habitat to supplement a 1998 acquisition that was designated as a preserve. To determine additional preserve acquisition area, identify critical habitat, and recommend management techniques, data regarding the range, habitat use, and seasonal activity of turtles at this location were collected through radiotelemetry, aquatic trapping, and drift fence captures. Eleven adult mud turtles (6 males, 5 females) were fitted with radiotransmitters and tracked between 12 May 1992 and 6 June 1993. Radiotagged turtles occupied ephemeral ponds between mid-April and late May, and aestivated in sand dunes in June and July. Some emerged for a second aquatic activity period in the ponds in July and August. Hibernation occurred in sand dunes from September through mid-April. Aquatic trapping and drift fence captures of adults and juveniles confirmed similar schedules of activity, aestivation, and hibernation observed in the telemetered individuals. Drift fence captures of hatchlings indicated emergence from nest locations from early May through June. A lack of hatchling captures in mid-summer suggests that hatchlings remained in ponds or dried pond beds through the summer. Important aquatic habitat identified during the study included shallow ponds with soft, muddy substrates and dense emergent vegetation; important terrestrial habitat included sand dunes with an elevation of at least 15 feet above the ponds. Telemetered adult mud turtles burrowed into sand dunes at locations of up to 90 m from pond edges. Females oviposited at nest locations up to 70 m from pond edges. Nest predators included coyotes (Canis latrans), striped skunk, (Mephitis mephitis), and western hognose snake (Heterodon nasicus). The recommended area for the preserve addition includes a buffer of terrestrial habitat to a distance of 90 m from maximum pond level edges. Additional preserve design recommendations include constructing a bridge or culvert under a road that bisects the site so that turtles can move between aquatic and terrestrial habitats on each side of the road, and encouraging native shortgrass prairie vegetation in acquired areas. Other recommendations include periodic introductions of hatchling mud turtles from a nearby, appropriately identified population, predator management techniques that encourage hatchling recruitment, and conducting habitat management activities with the potential to disturb soils between November and March.