A small enclosure was developed for isolating areas of submersed weeds within bodies of open water and for evaluating the efficacy of herbicides. The enclosure can be easily dismantled and transported between lakes and can be moved readily by flotation as a unit within a lake. Its use will permit smaller research plots, less herbicide per treatment, more plots within a uniform environment, and more accurate assessment of the relationship between herbicide concentration and length of exposure than are possible with the more commonly used techniques.