A microscopic study was made of 216 waterlogged wood samples recovered from an excavation at 500 Boylston Street, Boston, Massachusetts. Radiocarbon dating indicated that the specimens were from 5000-4000 BP. Most (206) of the samples were identified to genus. From a study of the sources of the sample, taxa used, and felling season, it is concluded that these materials were used in an ancient native American fishweir, a fencelike barrier and trap for fish on an ancient shoreline.