An aquatic system is described in which marine biota can be exposed under natural conditions for indefinite periods of time to dose-determinate levels of microwave radiation. A source of 2450-MHz CW energy is used to feed a standard-gain horn that radiates a glass-faced aquarium in the far field; source and aquarium are located in a microwave-anechoic chamber. Seawater in the 95-liter aquarium can be maintained at a desired temperature by a high-capacity heat-exchange system. By caging sets of one or more small aquatic animals at various distances from the glass face, determinate SARs at different levels can be achieved simultaneously within the aquarium. Reflectometric data on a small marine animal, the mollusc Aplysia californica, reveal that its presence in seawater does not appreciably change the load that is seen by incident RF waves. The system should prove useful in studying effects of long-term microwave exposures in animals that are otherwise maintained in a natural environment.