The sedge <em>Eleocharis acicularis</em> (L.) R. & S. is a mat forming species found in acid polluted waters with pH as low as 2.85. Both terrestrial and submergent forms are known and were shown by reciprocal transplants to be genetically identical and freely interconvertible. The colonies tend to be in backwash areas where water current is mostly negligible and substrate texture is clay-silt. Mats buried under silt are capable of reestablishing themselves by internodal elongation in association with the production of new culms. Measurements of the chemical environment varied widely but phosphorus and CO2 levels were thought to be of possible importance. Reproduction is most often accomplished by asexual fragmentation but seeds, when produced, have a germination rate as high as 86%.