Trials in Larimer County, Colorado during July and August 1984, with recreational screen rooms modified as large animal-baited mosquito traps are described. The two units tested are free-standing, portable and require no external support. In all-night trials, 462.5 mosquitoes/trap night were captured with horse bait compared with 367/trap night with CDC light traps. In 2-hour evening comparisons, mosquitoes collected per trapping period totalled 416 for horse-bait traps, 132 for light traps, and 93 for human-bait traps. Animal-baited screen rooms offer an alternative to existing methods for mosquito surveillance.