The Mill River, a stream about fifteen miles in length, located in Northampton and Williamsburg, Massachusetts, has been subjected since 1654 to disturbance by pollution, relocations of the channel, and impoundment for industrial purposes. Vascular plant communities in down-stream marshes possessed lower species richness and diversity and greater amounts of unoccupied substratum than those in upstream, less disturbed marshes. The invasion of the lower marshes by Marsilea quadrifolia L. during a period of heavy pollution was observed. Leersia oryzoides (L.) Sw., the only species found in all zones of all marshes, was dominant in early stages of floodplain succession. An inventory of the summer flora of more than 275 species of vascular plants within the river and its floodplain is included. Fluorescent strains of Pseudomonas were found at all sampling stations and comprise a part of the indigenous microbial flora of the stream. Coliform organisms, lacking at the source, occurred with high densities in all downstream stations. A method of enrichment for obtaining Pseudomonas species was devised.