Presents a detailed report on a study carried out in 1964-66, chiefly in Costa Rica and partly in Thailand, on the applicability of Holdridge's life-zone system [cf. F.A. 31 No. 4180] as a bio-mological classification for tropical forest systems, and its usefulness for aerial recognition of terrestrial environments (soil, vegetation etc.). Field research included vegetation and soil sampling, land-use investigations, climatic studies and aerial photography on 46 sites in Costa Rica representing 10 forest types of different moisture regime and altitude, and a trial application of the life-zone system to a reconnaissance ecological survey of parts of Thailand, including an outline study of vegetation associations in a smaller area. Though statistically significant relations between aerial patterns and ground features were not achieved, the life-zone system is considered a convenient and effective method for organizing environmental data in the American tropics, and probably also in other tropical countries. An ecological map of Costa Rica and life-zone maps of parts of Thailand are annexed, and an appendix reproduces the MEGA system of symbols for describing physiognomic characters of vegetation.