Due to increased deer/vehicle collisions involving endangered Florida Key deer (Odocoileus virginianus clavium), the Florida Department of Transportation (FDOT) planned to fence a stretch of U.S. Highway 1 that crosses Big Pine Key, Florida. Public access roads, which would allow deer to enter the fenced portion of the highway, posed public and wildlife-related hazards. Currently there are no structures (deer guards) that are effective in preventing deer from entering access roads. Our purpose was to design, construct, and test a deer guard that would allow normal passage of vehicles while preventing Key deer from crossing. Between September 1998 and December 1999, we constructed and tested deer-guard prototypes within a deer-holding facility at the Welder Wildlife Foundation Refuge near Sinton, Texas. Wild-trapped Texas white-tailed deer (O. v. texanus) were used as test animals. Deer-guard prototypes were subjected to 4 tests: (1) no incentive to cross; (2) extra food and water incentive to cross; (3) fawn separated from mother; and, (4) estrous doe separated from mature buck. Three deer-guard designs included: (1) a guard installed at ground level, (2) a guard raised (0.6 m) off the ground, and (3) a raised guard with sloped ends. Deer guards were tested at two lengths (either 3.6 m or 5.5 m) and had 1.9 cm cross-member spacing. We monitored effectiveness of each test visually and with infrared-triggered cameras for one week except for the fawn-separated-from-doe tests (2 hours). Deer jumped 3.6-m guards and walked through guards placed on the ground. No deer crossed a raised 5.5-m guard during any of the tests. With slight modifications, the design should be useful with other ungulates, including Key deer, and for use in urban environments.