The biological effects of various doses of radiation in plants are reviewed. Data are reported from a study of the effects of gamma radiation on a terrestrial ecosystem in which oak and pine are the principal tree species. Exposure rates around a 9500-C Cs¹³⁷ source varied from several thousand r/day within a few meters to about 2 r/day at 130 m delivered during a 20-hr day. Measurements were made of changes in the populations of species which formed the ecosystem, the rates of fixation, paths of energy movement through the system, differences in radiosensitivity among the plant species, and radiation effects on host-parasite relations. Results are discussed from the standpoint of the results of contamination of the environment with radioactive debris.