The applicability of two types of airborne infrared detector systems for censusing white-tailed deer (Odocoileus virginianus) was studied during different seasons and at different altitudes. This method of detecting deer was checked against ground censusing along an interstate highway that traversed the heavily wooded, mountainous study area. We found that detectability from the air was related to time of day, season, altitude, and wavelength sensitivity of the infrared detectors. In preliminary studies, individual cattle were easily detected from an altitude of 1,000 feet, using infrared equipment sensitive in the 3-14 micrometer region of the light spectrum. Deer in shrublands were detected at 100, 250, and at 500 feet during nighttime summer flights, using a 3-14 micron detector; the 3-5 micron detector was found to be a better hot spot detector and gave the best image during the summer surveys. In contrast, the 3-4 micrometer detector provided superior images during a nighttime winter flight. The interaction of seasonal and wavelength sensitivity effects were attributed to differential thermal radiation. During winter, the deer's hair surface is more nearly equal to the average background temperature than it is during summer; this would be adaptive in that body heat is apparently freely radiated during the summer and retained during the winter. Although of great potential value as a censusing technique for big game animals, current limitations of the equipment used makes large-scale airborne infrared detection of deer impractical unless the census area is flat and relatively free of obstructing vegetation. Extensive work with the equipment described here disclosed that nighttime surveys are more likely to be successful than daytime flights unless the sky is heavily overcast. Difficulties with interference from large amounts of reflected solar radiation in the shorter wavelengths of the infrared spectrum are likely to be encountered in daytime operations, especially on clear days. Refinement of the technique will undoubtedly provide a powerful tool for studying population dynamics and behavior of big game species.